Wow! That is a lot of oil loss. That would be the most pressing problem.
I will assume that you are using a good brand, and a reasonable viscosity ( 10/30, 5 or 10 or 15/40 etc) of oil. The first thing I would do is change the oil filter ( replace with OEM type, if not already) to make sure there is no problem with a defective filter causing excessive pressure ,stuck or bad bypass valve etc. Then clean the motor to determine where the leaks are. IF you still can not tell where the leaks are, you can obtain a pack of UV dye ( good local parts store) and a black light, follow directions and you will see where it leaks. Sure sounds like a rear seal, though. IN a big way. It would probably also behoove you to take it to a good shop, and have them ( if you do not have one) hook up a mechanical oil pressure gauge to determine the actual pressure at different rpm's. IF it is out of spec, there will be internal issues.