The overlap settings tell how a single image will overlap with its neighbors. 80% overlap means that any image will overlap with 80% of the next image. 60% of the next. 40% of the next and 20% of the next. If you do this in both axes along track and across track you will get 25 looks at everything.
The Grid mission settings doesn't change this. So if you do a grid mission at 80% overlap you will end up with 50 looks at everything (which is too much). The goal should be to get around 25 so it is safe to use a lesser overlap setting of roughly 65-70% overlap when doing a grid mission and you will still get a reasonable number of looks at everything.
It is funny you ask about the gimbal stuff because we were just having this discussion yesterday with someone. Must be something out on the internet these days... Mixing one pass of oblique images has been proven to be useful in linear mapping with high quality cameras. The northern pass was taken facing back across the rest of the passes. Every 4th image was included. It turned out shockingly well. It was with a P1 though so that might have something to do with it.
Aside from that we don't have much data or inkling to support the statement that mixing in a single oblique pass changes much.