Those stubs sticking up and capped are essentially worthless, and you can eliminate them in your re-pipe (unless they might have been for an evaporative cooler or something like that). If you have things that need hammer arrestors, put in engineered ones at the point of use (washing machine, DW, ice-maker may be candidates). Are the pipes already in the walls copper? If you are going to repipe, I'd not want to leave any galvanized around. Other than frying your brains, heat exhaustion, or heat stroke from working in the attic at those temps, any of the pipe systems should work. You really want the pipes to run right next to the ceiling, underneath as much insulation as you have. But, keep in mind, that insulation only slows the transfer, won't stop it. At the end of a long, hot day, if the ceilings are hot, the water in the pipes will be too. If you were willing to tear a little drywall out, you could make your joints inside the wall rather than in the attic, as pex would be able to make the corner without fittings. Since it is on a roll, it would probably be faster to run the pipe in pex.