Mark is business partner of Alfred. Jacob works as a stockbroker. So maybe Mark had some papers of Jacob wanting to sell the business. And Alfred was opposed.

Now this is an easy question. After carefully examining all the clues from mine wishlist I'd say that the answer is Arcanum. :)

Thx for a game, and for a fun afternoon. :)

It doesn't sound as an "undeniable proof of innocence" to me
But it's safe to assume that Alfred opened the library himself and led the guest with him. There would be no trace in the library fingerprint sensor other that Alfred's

Are you asking undeniable proof of Mark's innocence ?

How about fingerprint sensor at the entrance? Did Mark left his fingerprint when he went outside? So he couldn't be in the house when Alfred was murdered.

As for question three. I'd say Mark went to the pool to v-log. It's one of his, how you say, eccentric habits he picked up in an attempt to cope with the grief of losing his wife. The recording of himself out by the pool and the accompanying time stamp is the undeniable proof he intends to use in court.

Well, yes. Mark said: "I've the undeniable proof of my innocence but I will present it not earlier than on the trial!" I ask you to guess what is that he is talking about. And mark my words: he HAS the undeniable proof of innocence!

That's the one! Or at least very close! Actually he was trying to open the back gate in the garden as he intended to leave. He couldn't do that (the sensor was not programmed for his fingerprint after all) but nevertheless tried it several times and he did it exactly on the time of the murder! You won another 6$ Congratulations!


THE MYSTERY IS SOLVED!

At the moment I don't see any undeniable proof of Mark's innocence. As his business partner, he would know about the fingerprint sensors, of course, but like Ghorpm said: it's possible that Alfred took someone with him to the library and opened the door for the both of them.

Mark is probably behind supplying the police (or whatever) with documents of proof of Alfred's embezzlement. If that's true, I see no reason for him to kill Alfred, because he will already break his business partner down - and doesn't really mind that Alfred wants to stop working with him. Again no undeniable proof, though.

If he was the murderer, he wouldn't come back after he heard shouting. He would flee the scene as soon as possible. Still... the fact that he came back doesn't prove undeniable that he didn't murder Alfred.

[Edit]
Congratulations to the winners!

Probably Jacob was blackmailing Alfred about something, and Mark as a business partner of Alfred, knows something. Also, with the knife that fell into the pool, Mark has the necessary proofs that Jacob did it and not him (documents + knife).

To tell the true I don't know why Jacob kill Alfred... it seemed unimportant for the purpose of the contest so I didn't bother.

Nevertheless he did kill his cousin, put the knife in the secret passage, open the window and realized it's to high for a lame, one-handed man to jump so he rushed for the stair and was hit by Alison. The rest is obvious... or I should have said the rest is elementary ;)

EDIT: I will definitely make a similar contest in the future! I just need to create a nice story ;)

Thank you for this investigation contest