Demonic pact and Harmless Offering
Let's start with the basic question: can you make your opponent lose the game by "giving" them Demonic Pact after all but the last mode have been chosen?

Short answer: yes.



Demonic Pact's ability includes several uses of the words "you" and "your," which always refer to its controller.109.5  Harmless Offering makes your opponent the controller of Demonic Pact, so they must now deal with the Pact during their upkeep.  This also means that "You lose the game" refers to them losing, regardless of who used to control the Pact.

Another question to ask at this point would be whether the previously chosen options are "reset" when the card changes controller.  The Gatherer rulings confirm that they do not:


  It doesn’t matter who has chosen any particular mode. For example, say you control Demonic Pact and have chosen the first two modes. If an opponent gains control of Demonic Pact, that player can choose only the third or fourth mode.


Putting all this together: if you can "use up" the first three modes of the Pact and then give it to your opponent, Demonic Pact's ability will require them to pick the only remaining mode and lose the game.



Note, however, that there are ways for your opponent to deal with your clever ploy.  They could, for example:


Use a Counterspell to eliminate Harmless Offering before it resolves.
Destroy Demonic Pact using Naturalize during your end step.
Stifle Demonic Pact's ability before it can resolve.
Use Angel&#39;s Grace to prevent the Pact from making them lose.


All in all, though, it's going to be tricky for them to get out of this situation.  After seeing you play Harmless Offering, they'll certainly be wishing they could ignore the Gatherer rulings and "refuse your generous donation!"