Sincere thanks to @4cylinder
for the continued assistance, and to @ricksparts66
for the useful suggestions, and to everyone else who's helped along the way! Without your help I would be shopping around for used car and selling this one for scrap, but instead I'm here shopping around for a few more insights in hopes I can keep this one on the road. That's awesome!
I've been driving the car for a few days. Engine runs smooth and strong as ever, thanks to the brand-new plugs, belts, idlers, and engine mounts. (I stumbled upon a whole box of replacement parts in my garage partway through the project, including new engine mounts, and even a fresh valve cover gasket, so hopefully all the slow oil leaks are fixed too!) The problem remains that the engine sounds like it's about to self-destruct. The only reason I'm driving it is because I can't find anything wrong within the serpentine/timing belts, valve train, oiling, or lower end (at least based on examining the oil and filter for signs of bearing materials). But there's definitely still a problem -- someone flagged me down in a parking garage today to tell me there was something terribly wrong with my engine. Then again at the drive-thru. Preaching to the choir my dudes.
My plan: I think I can get access to a lift in a few weeks, so that I can examine the flexplate and/or attempt replacing the thrust bearing in situ. (I am sadly too old to do this type of work on my back in the driveway!) But I'll be calling in a huge favor to use this lift after-hours in a shop, while my non-mechanic acquaintance there will be stuck babysitting me with growing boredom and irritation. So I need to make sure I'm prepared to effect the repair. Of course I'll have no idea what I'm doing, but I'd like to at least make sure I'll have all the parts, tools, and supplies on hand. So I have a few more questions to help me prepare:
1. It looks like I can buy a crank bearing sets in various sizes. Is it correct to assume that I should buy stock-sized bearings, rather than any of the "undersize" options? And, based on catalog pictures, it looks like there's just one half-shell part that has a shoulder for bearing axial loads... so I would just remove one bearing cap and swap in this one bearing (and presumably its mating half-shell?)... right?
, you mentioned that it's difficult to source a flex plate, in which case I might need to track down a junkyard part or attempt repairing mine... However, it looks like there are two aftermarket flex plates available for this engine on Rock Auto, here is the stock photo of one of them:
Does this look like the correct flex plate for this engine? I'll want to have any an all potentially needed parts on hand, even if I don't need them, because I will probably have just this one shot at working at this shop with a lift and an engine hoist.
3. I have no idea how to go about replacing the flexplate. Would a Chilton/Haynes manual be helpful? Or, can anyone outline the general steps required... like do I remove the engine and/or the transmission? I am somewhat mechanically inclined, but have never removed a transmission or an engine from a car. Are there any critical tricks or pitfalls, where I could inadvertently destroy something or make a huge terrible mistake, even though I'd be working very carefully and trying to not mess up?
4. In removing the oil pan to access the crank, or removing the transmission (or engine?) to access the flexplate, are there any other parts I'd need to replace? For example, are there single-use bolts or gaskets? I'm assuming I'll need to bring all my tools, a fresh change of oil and transmission fluid, a new oil pan gasket+RTV, axle seals, and exhaust gaskets. Anything I'm forgetting?
I tried calling a few shops in the area to inquire about replacing the thrust bearing or flexplate. Not surprisingly, none of them seemed interested in performing speculative labor-intensive repairs on an aging, largely worthless car -- particularly one owned by some random idiot who's already tried and failed to fix it himself. It seems my best (if not only) option is to double down and hope I can finally fix this myself.