1. What is the advantage of restoring the DC BAT bus <100kt?
Both BATs are connected to DC BAT BUS and therefore DC ESS BUS, which is the important one.
Which also answers your q.2.
If you think in terms of FLY - NAV - COMM..... after landing..
Fly - stop on the runway or very close to it, for which you need power to ABCU, BRAKE PRESS indicator and Park Brake.
Nav - no external lights - no taxy.
Comm - VHF 1, FLT INT, CIDS, EXT HORN
Why do you need ECAM? The ENG FIRE lights still work, but not many pb's.