When the plug under the main cap is left out it doesn't send all the oil out to the filter. I usually don't take that one out as you can get to both sides of it.
I agree, but I usually take it out in order to enlarge the passage above it where it bottlenecks underneath the sending unit.
The threaded plug that is at the end of the drivers side lifter gallery should come out so you can get it clean. If you forget to install it you'll have very low or no oil pressure.