tought luck mate

does sound bad mate, could be a worn pump, or worse, worn shell's now, decreasing oil presure only points to one thing really, the pump, so unless the spring has failed, i cant see it being anything else,

how old is the engine? your still on the original bottom end arent you?

sump cant be removed without the crossmember being dropped slightly or the engine lifted slightly as far as im aware, as you cant get the sump out from the pickup tube when everything is in situ.

Also regards the dizzy drive shaft, if its not working your fucked, there is no inbetween, either its working or its not, it can get worn, but if it goes past worn, the drive will no longer turn the pump, so that answers that.