Most skimmers I have seen support a threaded plug that can be installed to block the water flow. Pull out the skimmer basket. There should be threads in the pipe underneath. (You can buy these plugs at any local pool store.)
You also may need to plug the return lines. I have found the simplest thing to do is put a rubber stopper on the front of it.
Oh, and I highly reccomend you repair/replace the failing valves so you don't have to deal with this again next year.
PVC is a good choice, provided that it is properly installed and maintained. A proper installation includes shielding the PVC from UV light, which is the primary means of premature aging. UV light will eventually turn PVC into a brittle mess.
ABS on the other hand tends to not become brittle with sun exposure, but to deform and is generally softer. As a result, buried ABS pipe seems to be at a higher risk of crimping, impeding water flow.
In either case, you should paint your exposed pipe, which probably wasn't done, and is the primary reason you are looking to repair it. Personally, I'd go with painted PVC, because after it is protected from UV light, it is less flexible (less pipe collapse) has a wider variety of fittings, and generally has a wider range of temperature distributions. I'm also more familiar with PVC, which might cloud my judgement. I'd love to hear from someone intimately familiar with ABS.
Both types of plastic are generally considered for low pressure applications, if you are having consistent freezing problems, neither pipe is meant to withstand routine freezing of water in-pipe.
PVC UVR is UV light resistant PVC. Buy it and paint it if you want a very long lasting PVC solution. Basically, you will have to worry about the (in my experience) natural enemies of PVC pipe before worrying about aging, that is careless yard crews, people vigorously digging, etc.
Best Answer
I would chip away everything that is crumbling and patch it. Make sure that you get all the cracks knocked out and patch out from there.
Chances are that the patch won't match the surrounding cement but if you think about it, you'll spend maybe $20 in patch materials (more if you need the trowel tools) and if it fails after a year you are out $20 and no harm done. If it holds, great, but at least you have a year (or more) to save for a larger fix in the future.
As far as leaving the rebar there or removing it, you should be able to leave it there with the proper patch cement. They usually have bonding chemicals in the cement and can bond to the rebar without using a chemical bonding agent. You may need to make a form for the underside or just mix the cement a little dry. Forms can easily be made from foam insulation (the pink sheets) available at any hardware store.
One more thing, when you patch, don't fill the expansion joints with cement, use duct-tape or weather stripping to keep the cement out of the expansion joint.