1: Yes if your team has good amount of night ops persons, uses best nvg in perfect condition
and has the occasional light sources when required, aka chem lights for everyone
G2 and H2 offer huge advantages at night, G1 and H1 at lower degree,
while for the mine sector the side from which you enter may count more,
from the north with the reduced space and good enemy cover its certainly easier at night,
while the eastern border of H3 appears the best side for daytime attacks,
the cover is so dense there that good night vision does not matter that much.
Some thoughts aka wall of text,
take in mind i usually play 1.13, but tried to keep its additional features out of the evaluation.
Grumm mine ist the most important sector out of Meduna,
so the queen will probably decide to send troops there after "grace period" ended.
Additionally the patrols from south to north will pass it,
so I recommend to have at least cleared I3 before you take H3,
(respectively for 1.13 have e.g. Elio stay in H3, as a scout he can "see" forces in adjacent sectors)
while the rest of the team continues to roll through Grumm.
However patrols moving through H3 will force you to rush to the defense anyway,
so its appears more effective to conquer the other sectors 1st and H3 as the last, at least if you come from the north.
G3 is often occupied by a medium force, I like to avoid any dmg, resource loss/weardown etc
therefore enter Grumm via F3 -> F2 -> G2, and continue counter clockwise.
(btw you can go G3 -> G2 directly if you hold Shift)
Keep in mind the long travel time for F2 to F3, i.e. have the required time to rest&restore 100% energy in F2
until movement to G2 is orederd so that we arrive at ~ 21h 10min, when the daylight is pretty much gone.
You should have some chem lights or other at least medium time light sources,
and either 2 grenades/explosives or wirecutters are required.
Original JA2 sector H2 has iirc no soldiers outside the fenced area,
so sneak when you enter the sector and use camo on the front persons, go prone 20 tiles away from the fence,
approach either north or east corner of the fence, clear it from enemies, preferably silent,
if not successful light up both sides of the fenced path and wait for targets behind the fence,
else continue along the fence until you are in line with the big NW to SE road,
place mercs at same distance to the hole you are going to create with cutter/grenade.
(here the most soldiers will appear over the time, probably it is easier for the games pathfinding to get here
and its easier to defend that entrance is you are forced to heal and/or repair stuff))
Either lay down at max view distance and stay silent, the few soldiers with script "explore" (or so)
will eventually try to occupy the space that you created for them by changing the maps path condition,
and can be killed without raising the alarm (= game goes to real time after soldier is killed)
Btw they will not do that if you close the gap with a merc or the bliue flag of an identified mine.
Optionally fire a loud shot, with most mercs at ~ 20 tiles from gap in fence and throw a chem light near it.
Wait to go inside the fence until you are sure all non stationary enemies are dead.
A night ops merc may also be able to take out some guys running around the lighted containers, from outside the fence.
For H2, secure the outer entrance to the factory with mercs placed > 15 tiles away from the brightness,
so they dont see the enemies inside the fenced area, else move back, we dont want alert yet.
Btw in case too many will rush the exit have grenades, but make sure not to damage the fence and create new exits!
The rest, preferably camoed night ops persons with silent weapons, clears the ACA building and surrounding area.
from up to iirc maximum of 3 soldiers for vanilla JA2.
If the remaining enemies are still unaware of our presence, they advance the fence in along line in prone stance,
preferably start firing at soldiers in the northern part to keep pressure on the main exit low, as alarm is pretty unavoidable now.
Soldiers will open the gates, it is crucial to interrupt them so the gates are not closed,
so you can later look inside and spot the campers who will not move at all (unless they see an enemy)
G1 and H1 have plenty of artificial light and full cover terrain aka buildings,
good basic advice is stay in darkness, have no cover for enemies near and bright area around you in little distance, as much as possible,
increase that with chem lights if required. And try to stay away from road and other zero camo tiles.
Also muzzle flash triggers interrupt & return fire,
so you can use the merc at long distance to draw return fire with low chance to hit, then fire with closer mercs to really hit.
Thank you very much for your answer and for your many useful tips.
In fact I don't have any mercs specialized in night operation, nor chemical lights, but it went well, for the record I had already finished the game in "Novice" and now I'm retaining it in "Experienced".
what really bothered me was that as soon as we took a sector the "grace period" was quite short.
Thanks again for your help. :)