1: You can't make an engine bigger than 5 MSP, so for some wierdo giant missiles, you may need multiple engines. Otherwise, no good reason.
2: I don't actually know what your techs are, so I can't say.
3: Yup. Unless you intend to shoot them at ships, you're best off with top multiplier and very little fuel.
The general AMM to-hit optimization formula is E = 5/A + S/2, where E = engine size, A = agility tech, and S is the space left over after you subtract warhead and fuel. If I had to guess, you might be better off with 0.6 engine instead of 0.5.
Erik> That has to be the worst designed AMM I have ever seen. Congrats.