On Hayward skimmers, the ones for liner pools have faceplates and gaskets, screws and also a lip on the sides and bottom into which you can srcew through the panel and secure the skimmer - the crete skimmers don't have any of this, because it's not needed. I don't know about the admiral ones, but i would certainly double check that the one you ordered has the faceplates and gaskets!.
Positioning of the returns depends on where the skimmer is located, if you'll tell me where the skimmer is (ie, center of the pool or 5' out from the deep end corner... etc), I'll tell you where to put the returns. The Hayward SP1408 through wall returns have 2 1/2" threaded shaft so the hole should be ~2 5/8" - large enough to get the return into the hole, but not so big that the flange doesn't catch. As long as the walls are still structurally sound, a little bowing after 30 years is ok. As long as you are adding new returns and a new skimmer, I would definitely pressure test the lines unless you plan on running all new plumbing.
Good luck with this version of the new liner! Get back to me on the skimmer and I'll tell you where to put the returns.
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