First, you need to quit going to those mechanics. Find someone who understands oil galleries and where they run in the engine block.
As RCP said, the only one of those listed reasons that could make any difference would be the oil filter adapter and it would have to be a leak which was a constant stream. Now, if you do have a constant stream of oil (and by constant stream that means with some pressure when the engine is running) then you most definitely need to take care of that ASAP. I rather doubt you have that situation though. The oil consumption, the cloud of smoke while driving and the puddles wherever you parked would have already driven you to the point that you knew you needed to fix that problem.
Follow RCP's advice.
If you still have any doubt as to the accuracy of the oil pressure then you can "T" in a mechanical gauge and compare readings.
If you still have low oil pressure you are probably looking at bearings.