[shadowarc]

Hasn't it been stated from the beginning that Sacred Gears use their owners thoughts, needs, and feelings to gain new abilities? I think that was said to be the source of Balance Breakers. His feelings towards his parents was shown from the beginning in his first fight with Vali (although his oppai power caused a bigger power boost) so it's a no brainer that something would happen again when they're involved. Whether it's Vali or Issei, when Ophis unlocked their DxD forms it also had to do with their mental state (Issei and his parents; Vali and his mother/step-siblings).

I agree the part with Pseudo Amrita felt a bit short but Indra stated that he was just providing a way for Issei to breakthrough that "wall" he's was facing i.e drinking his equivalent to lethal poison that even reached Ddraig. I'd rather not have to read a detailed 2-3 pages of "my body is being torn to shreds" even Dragon Ball didn't do that for Kid Goku.

I don't see how else you can expect a guy whose base specs are ordinary trash to compare to and level up the same as those geniuses who have been training and fighting longer than he has. It's only been 1-2 years since he was a normal high school student.