About the dagger to dagger mission, do you have a car? If you do, leave it at the bartender before you talk to Elizario. If you don't have one, there's a bug somewhere related to the dialog phrase. I think I edited the e6q.txt to say something like "leave your car and come back, but would have to check it again to see if I dint make a booboo and replaced that txt with another one. It has happened before
The mechanic only upgrades a few selected weapons, like in BE5. The weapons he upgrades are:
Benelli M4 Super 90
MP5A4
FN FAL 50.00
Dragunov SVD
M203B
I'll edit his dialog so he tells you this ingame, thanks for pointing this flaw out.