Please describe which section of cable became damaged again. You said it it was from the load to the sub-main panel. That is difficult to understand. It seems that you should have these components:
A. Service conductors. From the utility to the main 200 amp breaker.
B. The main 200 amp panel.
C. The existing feeder conductors to the 100 amp sub-panel.
D. The existing 100 amp sub-panel.
That is all that is clear from your post. Other things you have mentioned that are hard to imagine exactly are:
E. The existing 63 amp feeder. Is this breaker in the main panel?
F. The 63 amp load, probably not relevant.
G. The new 100 amp feeder. Is this breaker in the main 200 amp panel or in the existing 100 amp sub-panel?
H. The new 100 amp load.
I. The new 80 amp feeder. Where is this breaker?
J. The new 80 amp load.
My assumptions are:
1. Your use of (2) before the amperage simply mean a "double" breaker, otherwise called "two-pole".
2. The new 100 amp breaker was placed in the existing 100 amp panel (D). Is that true? What size wire/cable was used to feed the new load (H)?
Which cable burned?
With great power comes great resistance - times current squared, of course.