What should I replace next?

Randomly replacing parts is the least effective and most expensive way to diagnose something, as you're finding out. No two sensors are ever exactly the same. When you replace one, the Engine Computer has to relearn its characteristics by comparing its readings to other known conditions. That relearning will not take place when it knows there's a problem with something it is trying to compare to. Now you have multiple sensors sending signal voltages different than what the computer expects to see. That's why it gets worse as you replace more parts. Instead, if you're going to guess at parts, just replace one, then if it doesn't help, put the old one back in and move on.

I'm very much against having a mechanic tell you what to replace. He can intentionally tell you the wrong part to "punish" you for not just letting him diagnose it, or he can unintentionally tell you the wrong part because he is guessing from a lack of diagnostic test results. Unless there's a real common pattern failure that is seen all the time, guessing at parts based on symptoms is a real bad idea.

As far as the mass air flow sensor and fuel pump, it's not likely both failed at the same time. It IS possible for the pump to be not be building enough pressure but your description of the symptoms doesn't really fit.

The best approach is to let your mechanic diagnose it, but first, for a quick check, pull the vacuum hose off the fuel pressure regulator and check if there's raw fuel in it. If it's wet, replace the regulator.

I don't think I'd concern myself with the new parts yet. First get the on-going original problem diagnosed and fixed, then the relearning should not take very long. It would be the same as starting over from scratch after disconnecting the battery. You don't even notice when that is occurring.

Once it's running properly, THEN you can consider reinstalling one used part at a time. I'm cheap, ... Ahh, ... I mean frugal. To me it's silly to throw away good used parts while those new ones are becoming old too. Put the new ones on the shelf and put the old ones back to let them reach the natural end of their lives. The exception is if one of those parts had to be carefully adjusted, then I'd leave it alone rather than risking messing up that adjustment. Also, if a part is hard to reach, leave it alone.

Throttle position sensors cause very little trouble. When they do, it's normally a stumble or hesitation on acceleration, never rough running. The spark plugs are just general maintenance so leave them in. The battery is also a good idea. Since the '87 model year, GM went to a redesigned generator that causes a REAL lot of problems including running problems. They develop huge voltage spikes that destroy the internal diodes, voltage regulator, and can interfere with computer sensor signals. It is real common to go through four to six replacement generators in the life of the vehicle, but the best way to reduce the number of repeat failures is to replace the battery at the same time unless it is less than about two years old. As they age, they will still start the engine just fine but they lose their ability to dampen and absorb those voltage spikes. Replacing the battery every two to three years will reduce the chances of the heartache of a generator failure.