Thanks for the info, Bobbert!
Dpt, I believe the copyright notice should remain, since Legacy and Sentinel Comics are part of GtG's intellectual property. That said, in the versions used for printing, I think it's enough to have copyright on the front of the card only, just like the artist credit. That follows the format that GtG use in their printed cards.
I have made the incap power text bigger on the version that uses Peter's suggested power. I will now do the same for the other suggested power. Unless anyone has any further suggestions, I will just do a few little things to improve overall quality and leave it there.
I finally finished it! I'm happy with this as the final version. I made the size of the text and power box as close as I possibly could to standard printed cards. Then I tweaked a few aspects of the art, especially the crossword, e.g. trying to make the handwriting look a bit more like the front side, and fitting in a few more Baron Blade-related words (though this mostly can't be seen behind Legacy). As per my previous comments, I've removed artist credit and copyright, solely because printed cards don't have that on the back, and with full admiration for Adam's artistic talent and GtG's intellectual property. There's a full album with both versions here, the regular powers suggested by Peter and the modified powers suggested by bobbertoriley.

This looks absolutely incredible.
You made it possible, Peter! Thank you. 
Ha! Looking at the crossword without Legacy in front, I notice that "Timcosing" is in there...! Nice touch! :)
Had to fit that in there somewhere! :D
How did you manage to get those considering it seems you've been doing some worldly traveling lately?
Hahaha, looks like Christopher got that thing I sent him. 
I really loved watching this all play out even tho i didnt have an account then. I -snapped- at the morse code thing, but I loved showing my friend (another SotM fan) a month after it was done. And I quote: "Aaagggghhh. This is soo haaard. I love it! Wait what about Obliv"-tiptaptiptap. Thanks Peter forcreating such an awesome ARG and for the look on my friend's face when he solved it. He hated himself for not thinking of it sooner. His face was a lot like Cleverest Legacy's as he yelled "Goddamnit!"
Also kudos for everyone who made incap sides.