You might have to tweak both, high and low. But it's a bit hard to get the full context from the images shared. With unwanted projections from adjacent geometry, I'd look into Painters "match by name".
Every hard edge needs a uv split, yes, or you will get the "black edge" errors there. Every edge that is hard, or on the border of two smoothing groups, whichever terminology you want to use. Again this is why simplifying your low(and possibly high) makes a lot of sense here. You can also use a blend of hard edges and edge…