1. NSU requires one but ppl have called and got that waived. No other schools come to mind atm.
2. Probably very little especially compared to a science prof's LOR
3. Very likely not at all
I have 5 LoR's: 1 MD, 1 DO, 1 genetics (science), 1 epidemiology (science), and 1 human bio capstone (science)
I believe I am in same position as you - no real non-science LoR, although maybe 1 of those can be considered non-science since all are not "hard science courses".
So would a letter from my research PI count as a non science LOR? I'm currently having a little trouble obtaining one from a non science prof so I'd like to see if I can use that letter instead