For one reason or another, I managed to stumble into every unwinnable state the game could throw at me:

First, I was so good at avoiding casualties that I managed to attract too many merries in my team, as a result, when Maid Marian finally joined the team, there were too many merries in Sherwood already for her to appears on the map, I had to send twenty or so merries to their death or on scouting missions before she finally appeared in Sherwood.

Then, I got to the point in the campaign where Robin is arrested, but somehow the game was only offering me The Black Castle mission, a mission that requires Robin to be present. So I had to load a previous game and deal with that mission before getting Robin arrested.

Now, I got Robin arrested, but the game stubbornly refuses to offer me the Escape mission where you must rescue Robin. The only thing it offers me now are ambushes missions.

I noticed that story missions only become available when you've acquired a certain amount of money, for instance, the archery contest mission only becomes available when £60 000 have been raised, and the wedding mission when you've acquired £70 000. By the end of the wedding mission I was already midway into the £80K, and that mission where Robin got arrested bumped me midway into the £90K. I thought acquiring the full ransom would unlock the following mission, I was wrong. Then I thought "You know, maybe you've raised too much cash, maybe the mission is only available when you're between £70K and £80K", and so I reloaded the wedding mission and gave close to £20K to a beggar, it took me ages because it was at a rate of £50 every five seconds. That did not solve the problem either.

Just in case the problem was something I could have done in an earlier mission, I restarted a new game from scratch, using the cheats to quickly get to the point in the story where I was before, that didn't help, the game just stubbornly refuses to give me the Escape mission.

So, I was wondering, if somebody had a savegame in Sherwood forest, where the mission screen allows me to select the Escape mission so I could finally finish that game, that would be great.

I do not really care about the percentage of soldiers left alive, nor the number of merries in the gang, nor how much of the ransom has been raised thus far, now how trained the merries are, nor the progress on the war front. I am playing the French version but if you only have an English savegame, I have no problem downloading and playing on the English version of the game.

The only three details I care for are:
A) I want the savegame on Hard difficulty (Sadly the only savegames available on the internet are Medium difficulty savegames)
B) I'd appreciate if all six treasures (The royal scepter, the Domesday book, the Coronation spoon, the silver arrow, the Sword of State and the Ampulla) available up to that point were collected
C) In the wedding mission there is a beggar that asks for £2000 and should you pay him, he'll reward you the next time you visit Nottingham, I'd appreciate if that guy was paid

I love the game, it's one of the few game that made me wish there was a level editor or a way to mod it, but damn, it is overstaying its welcome with all of these "the game is now unwinnable" pitfalls I fell into in the last three missions.

I've finally fixed my problem, it was either one of the couple of fixes I installed on the game to improve the slowness issues, or the French version was to blame, I dunno exactly which, but earlier during Xmas break I installed the English version and used the WINNER cheat code to see if I could reach the Escape mission and I could, so I restarted the game from scratch, to do the sidequests and collect the six jewels of the crown trinkets, carefully checking every now and then if acquiring the jewels or doing the sidequests would screw up my game but I did not get into any unwinnable state and I was able to go forward into the plot.

The English version reads v1.1 on the title screen, whereas my French version, after installing a bunch of fixes to improve lag issues reads v0.0. The only fix I installed on the English version was the one that involved copying a file named aqrit.cfg and a file named ddraw.dll into the game folder. That was the only fix I installed in the English version that actually fixed the lag issue but didn't break the game.

Anyhow, if anyone has any question regarding the mission order, or had any doubt that their game got broken or stuck in an unwinnable state, or if anyone is interested in having savegames (english version, hard difficulty) at every mission in the campaign, feel free to ask away, playing this game over and over again because I fell into so many weird bugs gave me some insight at how the game should normally unfold and how it can abnormally unfold.

I received a private message this morning from someone saying they experienced the same issue as I did regarding the Escape mission and requesting my English savegames but that person has his or her "Receive private messages" option set to Off and so I can't directly reply to him or her, and so my only option is to post the link to the savegames here and hope he or she will notice it:

https://www.mediafire.com/?tdtu9fg3e7r32q8

Here are the savegames, simply install the English version of the game, extract the content of the Savegame folder into the GOG Games\Robin Hood - The Legend of Sherwood\DATA\Savegame folder.

You'll be playing on Hard difficulty, all the savegames are made after the mission has been selected but before the team is selected. I did my best to level up the fighting stats of the merry men but with the exception of Stuteley, and a merry of each type, none of them have maxed out fighting stats because I kept taking Robin, John, Scarlet, Tuck and Marian on missions. I believe the hideout map should be full of items to replenish yourself in between missions. You've got ten clovers and all the six crown jewels that were available up to this point. Simply load the Escape mission savegame, read the briefing, pick your team and you should be good to go. There is also a savegame for every ambush missions and one for the Liberate Lincoln mission for the obsessive completists among you.

The fixes to fix most of the speed issues can be found in this thread:
http://www.gog.com/forum/robin_hood_legend_of_sherwood/when_will_gog_update_this_game_for_windows_8

Good to see that someone has already found that and he/she has a fix for it. But I am still confused, what is the reason behind this soft brick? Did it happen for using too many cheats, for too many merry men or my Windows 10 (i am using DXDnd emulator for compatibility)? & I don't want to use other savegames I want to play on my own. So can you give me a fix for it or just let me know the reason behind this brick I will try to fix it myself

I managed to get the Escape mission but for some reason Maid Marian is crossed out. She is missing in the Sherwood map to be selectable either.

EDIT : Wow so merries need to die for her to appear brilliant game design by Spellbound

Finally finished the game, it was 20 year old backlog that needed to get out of the way.

And what horribly glitchy/buggy mess the game is made worse running on modern systems.
What should have been a 15 hour affair was turned into 30 hours due to to game messing up the progress and requiring multiple restarts.

Considering the age, most likely the devs have long moved on and support is no longer a possibility.
Therefore I am making notes here on my end if other players hit the same unwinnable state or brick for posterity on top of what the OP has pointed out.

1. There is no point keeping too many Merries in Sherwood, it will cause additional buggy Little John's to spawn in Sherwood and Marian will not spawn later as mentioned above by OP. I had to load an earlier save and send 20-30 merries to their deaths to keep the Sherwood headcount low for her to appear correctly.

2. The game does seem to trigger main missions based on amount of cash until The Escape, but it should not be doing this after The Escape and The Letter mission.

I had a problem the March on York (2nd last mission) mission did not trigger. The correct trigger would be 100k ransom paid in The Letter mission, and March on York would appear for selection with the balance cash in Sherwood. However I got countless ambush missions. I tried reloading the mission again per the topic here but I did not work

https://www.gog.com/forum/robin_hood_legend_of_sherwood/endless_ambush_missions

Worrying the cash cheat code might cause additional problems, I used Cheat Engine to modify the cash to a very high value ~200k when on a ambush mission, and lo & behold the March on York mission was available when back in Sherwood.

Try not use the use the cash cheat code too much OR have too much cash before the Escape mission. This might break the game as well.

3. Save often as possible in every mission with saves at the beginning and end of them. Might never know when you might hit a brick state.

Side notes:

There is not much guides out there to get all the 7 Artifcats to give the Secret Level or Ending.
So someone has made a video (its in Polish but straightforward to follow). Putting it here.

https://www.youtube.com/watch?v=YhPkLpiGHPY

Good riddance, safe to say I'm done with this game.