1: I'll leave that to a ballistics expert!
2: Type one has the most lightening cuts to it, basically just metal cut away in non-critical areas to make the receiver as light as possible. This gives it it's distinctive look and contours. The type two is a modified version of the type one with fewer/different cuts. This was brought about because some of the earlier type ones were cracking under sustained full auto use. The type three essentially eliminated the cuts as a money saving move. Carry handle cut or not is mainly cosmetic. Accuracy is not match grade on most FALs anyway, so not much difference in receiver type regarding accuracy.
3: Not a metallurgist, but the type three is probably not much/any stronger than a type two. Hot loads will probably loosen the locking shoulder before any thing else. Also, many receivers are cast instead of forged. Forged will give more strength than cast.
4: Entreprise and Century are probably the two receivers that require the most tweaking. The last batch of Imbels was not much better from some reports. The new DCI run looks promising. The DSA is still the best though.