Key Used

No, he has to give you the same game you won, not a different one. And you can't marked as received the other game, since you will result with a game won, marked as received BUT not activated.

if the game is changed, the game giveaway must be changed by steamgifts staff.
gifter have to open a ticket for change/delete/reroll giveaway. I think gifter has to show if you are ok with any changes.
you only mark as received what the giveaway indicates that you won.

If you agree with it, he can delete the GA. If you mark it as received, it will look like you've not activated the key, and you'll risk suspension.

Yes, you would have to receive the same game that you would win through the giveaway, or you will get suspended for not having the game in your steam account but marking it as received.

Yes, there WOULD be problems. I had this happen to me, as the giver. The game has to be the same because otherwise it'll show that you didn't redeem your win, because you don't have the game in the account. What my winner and I did was contacted SG support; they deleted the giveaway and as a gesture of my apology, I gave him a list of available keys I had and let him pick one that best suited his interest.

if you mark as received but dont have the game in your library, you will eventually get a temp ban and if you are on the forum wont be able to enter many ga there because the creator places many times a filter for it on the ga link.

Besides the ban and the great chance of not being to enter forum ga. Creators of your future wins can request reroll on that notion.

I assume another part of the problem with this is that he'll be getting value credit on his account for something he never gave. Might not seem a big deal but it could be abused to gain advantages.