Shah:
1) t=1.5*td where t is the material thickness and td is max c/s depth. therefore your answer is 2/3
2) I would go with Genghis on that one.
3) I would say weight reduction, from design point view. This is a general idea when they teach you or better re-teach you when you start working for one of big manufacturers. Machined is always better then a lot of sheet metal attached with a lot of fasteners. Too heavy.
Also, not to forget, machined structure makes stress engineers much happier
4) Same as Genghis although I would like you to say that force (or moment) is what acts on a beam, not stress.
5) Bonding is your answer to eliminating knife edge
I hope it helps.
Cheers