1.- I use young FMs instead of cavalry in my legions and occasionaly to command small detachements to surpress rebels and random enemy units. In battles the senior general usually just commands, while the youngsters do the cavalry fighting. Thus they usually gain several silver chevrons before becomming mature and a generalship or governorship of their own.
2.- I do not have Alex, so I d not know. As Roman I play on H/H on RTW.exe
3.- I have never needed to attack anyone (the script did that for me in the First Punic War) all my war were pure "self defense". Well I did attacked others several times for the purpose to maintain ballance or protect my allies. For instance when Lusotani, reduces to Potugal and Pyrennees after a war with me changed their direction and started to attack my trustful allies Arverni in Aquitania. I have twice took the city from them and gave it back to Arverni, the third time I have finally destroyed Lusotanna altogether. Generally, the more you allow AI to evolve itself, the more interesting battles you will fight and the more rewarding the game will become. Your blitzing is your greatest enemy!
4.- I have indeed installed Allied governments in all conquered settlements and replaced them with Type III (and proper Roman FM as governor) once the allied ruer died. I will replace them with Type II in another 20-40 years. In Syracusae, Massilia and Emporion I will maintain allied governments forever.
5.- Allied generals lead only troops from their own province (i.e. non-itallians) as homeland defence, or (occasionally) as an independent national undertaking. (Such as when Massiliotai - allied to Rome by Type IV - regained their former colony Emporion)