A couple technical points -
1) I'd specify Hunter's Mark lasts until the end of your next turn, as per the errata.
2) I'd specify that the warlock gets the at-will that matches his pact. Also, this might best be limited to "till the end of your next turn" like HQ.
I agree with Larry, though, that swapping at-wills might be too much. What I think might be better is the "choose an at-will as an encounter" like the Wizard is RAW. I think it's great you can multi and get a wizard power, but why can't you multi and use Tide of Iron once a fight (per RAW)? Your total swap goes a bit too far, I think, but as an encounter I think it works very well.
I like the addition of more class features and requiring a skill be trained as a pre-req.