That's not a megaproject, that's a Gigaproject! (At least!).
I dunno how you'll get enough adamantine. and how you'll finish it with one dorf before he die of old age. (You do plan, on leveling him to get only masterful floodgate and such, right?)
For the design, I'd say a Plump Helmet. As an tribute to all the Engraver that survived an attack of clown and still didn't produce anything interesting because of RNGs... And for the WTF.
For the weapon, hammer or axes are must have. I suggest hammer, since the square side will allow for cooler magma drop than an axe's edge.
Also, you really plan on making him along human proportions? We're playing Dwarf Fortress, not Simcity!!! and don't forget the Silver Beard of Wisdom!!
As for the rock, I have the following suggestion:
1) Don't touch the topmost z-level.
2) Excavate the other z-level (You may want to dig your map in 2-3 step, to keep your FPS high
3) Channel all the edge, leaving only stairs between level and a support supporting the whole she-bang.
4) Link the support to a lever safely placed under an intact layer.
5) Pull it, the cave-in will destroy all the stones.
6) Profit!
(Now that I'm thinking of it, I'm almost sure that you don't need to let the topmost layer intact, just leaving floor should be enough to destroy the stones. I'm only 99.999% sure tough)
Oh, and if you stop playing hermit, be sure that all your dorfs are in safety.
P.S. And you may want to mod your dorfs with an insane speed ([SPEED:1]), [NO_EAT], [NO_SLEEP] and the like...