You young whipper snapper!! I got tired just
reading your post!!
Can't help you on most of that, but if you're willing to dig enough to replace returns, here's some thoughts.
Consider the location of your skimmer, and/or adding another. Has it been working OK? Skimmers should be located to take advantage of the prevailing wind, so that surface debris both circulates to the skimmer opening, and gets blown to it by the wind.
With a properly designed skimmer and return design, you don't need a bottom drain at all, so you could kill two birds. Drains are ugly, hang up vac's and brushing, and can be a safety issue (entrapment). No drain = no problems. Especially something to consider if you only have one drain. There are drain systems that can address some of those issues: adding a two-drain system, or a flat grate-type drain, etc. But those solutions wouldn't address the older plumbing.
I thought I read here that black poly is a very reliable material (but you have to confirm that, don't take my word for it), so maybe just leaving it all alone is the best solution. Something to research.
Do your returns have directional eyeball fittings? Whether you redo or not, be sure you end up with those. Mine were added to my returns during my resurfacing. Deleted my drains at the same time. No regrets, the pool circulates just fine. Now, my pool is only 7.5' deep, so 9' might be something else again. But even adding a low return would be better than a drain, for circulation, IMO. The trick is to find someone that can engineer your circulation system for optimum performance, one who could determine for you if your current return layout would be sufficient without the drain, or what you would need to add or move to improve performance. Can your PB do that? Or maybe find an engineer that specializes in pool design to give your system a once over. I wouldn't think that would cost all that much, and this is not something you want to guess about, or get wrong, since it'll all be under brand new concrete eventually.
If you add or replumb returns or skimmer(s), I'd want each to run back to the pad, on their own dedicated valve, so that you could really dial each in.
There is a school of thought here that pressure-side cleaners are old-school tech, and not to be a first choice option. They still make 'em, and install 'em, but robots are all the rage. I wouldn't want a robot, so I used the middle ground. I had my pressure line converted to suction and replaced my pressure vac with a suction vac. To be honest, I didn't see a big difference, but I was able to eliminate the booster pump, which helped with the energy bill. My pressure vac (like most? all?) had that stupid tail that sprayed out of my pool and ruined my windows. Took me two days of mechanical polishing to remove what that tail did to my glass!! Look into robots. Research pressure vs suction vacs. At the very least, I'd make sure the new plumbing can be used for either type of vac, so that you have the option. Even if you go robot, I'd still install that line. Ya never know when you might need it. If nothing else, it's a convenient connection for manual vacuuming for the quick spot cleaning jobs. That all said... Warning: a suction port poses an entrapment danger if not properly plumbed (needs a special flap device), so there's that. A modern VS pump (any pump, really) and an unprotected suction port can eviscerate a person! There's no coming back from that. Like a drain: the safest suction port is no suction port.
If I was replumbing everything, I'd use sweep 90s exclusively. Many here think that idea is hooey, and that a sweep 90 doesn't provide significantly better flow, not enough better to justify their cost. I don't agree, but maybe that's just me.
OK, that's all I got this morning...
