Dear Brother , i am currently using OSRAM BILUX 60/55w in my car with relays and ALHAMDULILLAH i'm satisfied with the results . As you mentioned above, the calculations you provided are right!
using a 100/90W bulb isn't really dangerous for the wires, the main difference comes with the BULB HOLDERS, more the current passing through it, more they will get HOT and eventualy the low quality bulb holders MELT DOWN.
For the RATINGS OF WIRE, i have no idea but the wires are good enough to bear load upto 10amps (as per my idea) i can be wrong too.
Why dont you try to have some good quality bulbs of 55/60w (adjust them properly ) and add relays with them (i am using separate relay for low beam and separate for high beam)
Regarding your last question, the car is okay, needs some serious care as i was busy in some other commitments and younger brother is busy in his Mid term exams. Will get tuning done and thinking of installing EFI setup to this engine.