Regarding 3, the ability to have sex with something, and the ability to reproduce, are two different things, just because you can screw an alien, doesn't mean you can have children.
I meant like...genitalia position or whatnot.
I haven't played the game, but I'm willing to bet it's implied that the aliens you can screw have openings/protrusions in all the right places :P
Most likely. The asari don't reproduce like the other species, but it is implied at a certain point that krogans are very... Well equipped.
Also, am i the only one who finds it rather odd that all the alien species can speak perfectly good English? (With some accent here and there, but still)