Ouija,
I feel your pain. I had to do a lot of looking and asking before building my M4.
My recommendation: do not try to build the M4 as a be all, end all solution. Keep it simple and configure it to your needs. If you want long range effectiveness, build it like a National Match or Varminter, if you want CQB, M4 configuration with light weight barrel is awesome. Medium range, 3 gun match or all arounder, 16" midlength or dissipator handguard.
I initially had a light, but took it off and put it on my Mossy 500 for HD (I won't be clearing caves anytime soon). The grip bi-pod is mainly a tool for zeroing / target shooting, but I would prefer a shorty foregrip or GASP no foregrip at all. I like the Eotech, but I need to same up some cash for that, so currently, I'm usuing an Aimpoint clone (not bad for $50 copy)
My AR is mainly for SHTF and paper punching (using the Ciener .22 LR conversion)
I'll do my best to answer your 2 questions, but I'm sure that others will chime in with more experience and knowledge than I.
1. The 5.56 NATO is compatible with the .223 Remington. But usually not the other way around. The .223 has tighter tolerances (so feeding 5.56 might be finicky) and the 5.56 is designed for higher pressures. I've read that the Wylde chambering is somewhere between the two (ie: for accuracy) but RRA has a better description than I can explain.
Depends on the type of 7.62 round... the 7.62x39 is the AK/SKS round and the 7.62x51 is the NATO round that is very similar to the .308. I don't know enough about the similiarities or differences of the 7.62 NATO vs .308 to comment intelligently, so I'll shut my yap. With everything being equal, yes the shorter cartridge length ammo are more accurate but that's like saying a Suburban is faster than a Lotus Elise because it's got more HP... many other factors that go into that equation.
2. In generalities, carbine = assault rifle. I'm sure that others will have other opinions and cite historical reasons why they might be different. Carbines were generally issued to personnel not directly using the rifle for their MOS/jobs, but needed something with more reach than a hand gun. Artillery crews and tankers are prime examples. The term "assault rifle" is relatively new in the nomenclature of weapons, ie: getting more and more use as a term with the introduction of the M-16. Since it used a varmint round and didn't have the 1000 yrd. sighting / terminal energy as the prior "battle rifles" (M1 Garand, M-14) the term "assault rifle" became the de-facto name, among other obvious and less obvious reasons.
I'm also a noob, so if others know better, please share your knowledge.
Cheers,