As far the radar... you are right!
My idea is that Night surface attacks should become impossible by tweaking:
- radar setting (ai_sensors.dat/sim.cfg)
- bigger convoys (campaign.rnd)
- more escorts (campaign.rnd)
Also the campaign.rnd should be adjusted with spacing in convoy lanes to the historical correct 1 km etc. This make it a bit harder to compensate for the night advantage the u-boot has.