I agree with RAWalker (detailed first response) and RichKeller (response to all 4 points).
I, too, and a PSI/PCI inspector and have also seen all these questions well addressed in the documentation / training.
Now for some anecdotal evidence:
1) I have 5 steel 72s, the newest of which was produced in 1976. I bought them all used (of course, since I was only 7 in 1976) and in fact, the last 2 I just picked up this year. Some are lined, some unlined. All of them still pass hydro with + rating, so I regularly get them filled to 2475-ish (really 2500). 2 are doubled up and the other 3 I use for sidemount. They work great in both configs, have good buoyancy characteristics (compared with AL80s), and can take a beating. I just stripped the paint off the doubles and cold (re)galvanized them. I did this not because they had any rust (they were clean as a whistle under there), but just so they would match and look like new (had 2 different paints on them before). These are my favorite tank size because they are big enough, small enough, and dirt cheap (I think the most I paid for one was $50).
2) About 6 years ago I bought a set of 4 AL50s that were made of 6351 for $20 each on eBay. All 4 passed hydro, but one failed Eddy and was condemned. Still a pretty good deal ($80 to buy plus $50 to ship plus $80 to hydro equals $210. That’s still just $70 each for the 3 good ones). I originally bought them for the kids to learn to dive, but they never did take OW and now they are full grown so wouldn’t need a small tank anyway. For the 5 years I had them, I used them for sidemount (2x50cf is equal to what my single backmount buddies were using, so worked well). When they came up for hydro again, I sold the 3 for $50 each on eBay and bought the last 2 72s mentioned above. So overall, these tanks cost me $4 a year each, including hydro. Sweet.
3) I have not had a problem yet with my lined tanks. A buddy of mine had one where the liner was starting to peel, so we tumbled it and then O2 cleaned and dried it. It has held up fine in the 2 years since. Still clean every time I vis it for him.
4) See Eddy comment in item 2 above.