Pic attached. Looks like it's the 10mm allen screw.
So, i'm having trouble following the situation. Does this mean that the Momo adapter linked to above, WILL fit my vehicle on the column end of things? I guess i'm wondering about the spline business.

I guess if that works out fine, then it's just down to the 70mm bolt diameter, which i do believe is already settled with this steering wheel and this momo adapter.

As for the horn button... I'll take a look at the dual contact situation, but i'm assuming that can be modified easily to just be a regular SPST switch and not a DPST sorta thing. I'm assuming Mercedes simply doubled up on the one contact end of things so as to always guarantee action. I could maybe even install a second contact wire somewhere in the new wheel so that the vehicle follows the safety standard that Mercedes requires.