They don't unless you have a waiver issue, it is all about picking the people with the best chance of succeeding in the pipeline, the other designators have less rigorous training pipelines and will look at most things, the checklist is used for all designators. We just wouldn't send in LOR's and the motivational statements for my applicants would be 2-3 sentences, a paragraph at most.
ASTB is weighted significantly more than the GPA.
Nope aviation doesn't care, we would routinely see STEM majors with low ASTB declined, we would also see people with high GPA's and ASTB scores at 5-7 for PFAR get declined while people with GPA's in the low 3's and PFAR 8 or 9 get picked up.
Again it is all about picking those with the best chance of making it through the pipeline.