1) 5A 5B 5C 6C 214C isn't a combo unless they're crouching; if you want a very basic beginner combo that actually works do 5B 5C 214B. Look at your combo counter, if the number of hits resets midway through it means the combo broke and they could have blocked or hit you at that point. Also 5A sucks, you should start your combos with 5B always. There's also a thing called proration in BB which reduces the amount of damage your attacks deal as the combo gets longer, so don't do repeated 5A, it'll actually reduce your overall damage.
2) Icecar is unsafe on block, which is why people don't use it unless they know it'll hit (i.e. not from fullscreen). That means that if you do icecar and the opponent blocks it, they can hit you while you're still coming down from the attack. As for what the different icecars do, 214A is shortest and doesn't have the followup hit (also the safest), 214B is medium range and faster than 214C, which goes further and faster. 214D costs 25% heat but does a lot of damage and knocks them down so you can pick them up afterward with another attack.
3) 236A is a medium-speed projectile, while 236B is fast. 236C has a lot of startup (it takes him a while to shoot the fireball) but it also recovers the fastest. 236D costs 25% heat but is bigger, hits multiple times, and freezes the opponent if it hits.