LO,
To answer your points:
1. The fuel cells on the A and C are fully used - C of G is always a challenge on combat aircraft, and the more fuel you have at the ends the better you can control it - with some clever fuel scheduling.
2. A and B tails are miles smaller then the C's. Same shape, but very different size. IIRC around 35% smaller.
3. B needs an internal weapons bay for the same reasons as the A and C - bay to get the range/payload it needs, and also for LO reasons.
4. Yes, internal vs. external gun is a real poser - one could argue that fitting it only when you need it saves weight and gives more options - and integrating the gun they have chosen is a real challenge - heavy, takes up fuel space, adds drag (big bump) and getting an LO gun muzzle to work is not easy.
5. You are right that single engine doesn't necessarily mean cheap - but single engine and single seat is a powerful way to keep overall weight down - and that is still a major cost driver.
Regards, Engines