My impression is that the backer material I put behind the surround walls is more important than whether the backers overlap the tub flange or not.
Yeah, I'd agree with that, especially if you're gonna be using plastic panels where there's less chance of water leaking into the wall. Lots people just glue tub surround panels to bare drywall and those installations last a long time. A plastic tub surround is more reliable than ceramic tiling.
Several of the replies mentioned tile and it sounds like I should forget about gypsum, including green board, altogether and use one of the cement board types. Is that still the case even though I'm installing solid glue-up panels instead of tile?
You definitely don't want to go with drywall or greenboard, but there is a gypsum based tile backer board made by the Georgia Pacific company called Dens-Shield that's gypsum based, and as easy to cut and install as drywall, but is very water resistant and suitable for use as a tile backer.
http://www.gp.com/build/densshield-tilebacker-board
I used Dens-Shield to replace the water damaged drywall in my sister's finished basement after she had a mini-flood, so that now, she can get up to a foot of water in her basement without having any significant damage to the walls down there.
The Cadillac of tile backer boards is Hardie Backer Board, and you want it in the 1/2 inch thickness for better rigidity. I've found Hardibacker to be the strongest and hardest tile backer board, but I've also found it to be the hardest to work with and install, and it's also the heaviest of the tile backer boards I've used, and that makes it harder to work with, too.
If you're installing glued up panels, then you don't need the strength of Hardibacker as much. With ceramic tiles, any flexing of the backer board is likely to crack the grout lines in the tiling, and so that's where you'd be better off opting for 1/2 inch Hardiebacker as your tile backer board. That's not saying that the conventional cement boards like Wonderboard, Durock or a gypsum based tile backers aren't appropriate, it's just that Hardibacker is stronger (for equivalent thicknesses) and so there's less chance of it flexing enough to crack a grout line.
Is a moisture barrier still necessary with cement board? Poly? Liquid Waterproofing? Both?
Yep. None of the cement boards or Hardibacker are impermeable to moisture, so the general gameplan is to put up the tile backer board on your studs, then paint a moisture barrier like "RedGard" (which you can get from Home Depot) over the tile backer, and then tile over that moisture barrier.
Whenever you do home reno work, it's never a good idea to have TWO moisture barriers in a row. That's because if water gets between them (from an overflowing bathtub upstairs, say) then it'll take forever to dry out. So, if this is an exterior wall that your're doing, you'd kinda have to choose between putting up a polyethylene vapour barrier over your insulation OR applying a moisture barrier over your tile backer. In a case like that, and I was planning to tile, I'd probably paint the moisture barrier over the tile backer to block any moisture from getting into the tile backer OR the insulation.
If you're sure you want to put up plastic panels, then in my opinion, they'll be their own vapour barrier, and I would just glue them over your backer board. If you're installing plastic panels, I don't see nearly as much of a problem using drywall or greenboard behind them. But, if it wuz me, for the incremental cost, I'd use DensShield behind your plastic panels. It's way more water resistant than greenboard, and is actually overkill if you're putting up plastic panels. But, there's absolutely nothing wrong with overkill and I support overkill 100%.
For gypsum or cement board the surround instructions specify "To ensure proper adhesion of the Swan Corporation product, the wall surface must be sealed with a water or oil-based primer/sealer." I don't suppose that would be sufficient waterproofing?
Your plastic panels are sufficient waterproofing in themselves.
Lots of times people will stick plastic panels up directly over bare drywall, and that's what those instructions are telling you not to do. That's because no matter how strong a glue you use, the weakest link in the chain is going to be the paper fibers in the drywall face paper. If you glue something to bare drywall, you can pull it off and the surface of the drywall, that is, the paper fibers at the surface of the face paper will come off with that glue.
So, what they're wanting you to do is paint the drywall first to consolidate the surface and make it harder to pull the plastic panels off. If it wuz me, I would use an interior alkyd primer that'll dry to a much stronger film than a latex primer. And, if it wuz me and I was he11bent on overkill that day, I'd give it two or more coats, leaving the paint roller and tray wraped in plastic bags in the fridge (good) or freezer (better) between coats. And, put each coat on within 24 hours of the previous coat to ensure excellent coat-to-coat adhesion.
With fairly rigid surround panels can I get away with furring strips and 1/4" cement board to keep a 1/2" wall thickness, or would it be better to use 1/2" board, leaving a small gap above the tub flange? (the surround panels will come all the way down the 1 1/4" tub nailing flange anyway).
The latter. Use 1/2 inch board and leave a gap above the tub lip.
And, in case you want to know why, it's because one 1/2 inch thick board is more than twice as strong as two 1/4 inch thick board sitting one atop the other. That's cuz the formula for the deflection of a beam has the beam's height CUBED in the denominator. So, for example, a 2X4 being used as a floor joist has a height if 3 1/2 inches. 3 1/2 inches cubed 42.9. A 2X8 being used as a floor joist under the same small floor will have the same width, but is 7 1/2 inches tall, 7 1/2 cubed is 422. 1/42.9 is ten times more than 1/422, so given the same span between supports, a 2X4 floor joist will bend 10 times as much as a 2X8 floor joist under the same weight.
So, 1/2 inch thick board is stronger than two 1/4 inch boards. That's why you can bend a deck of cards in your hands, but not a piece of wood that same size.
And, that's also the reason why a fir 2X12 can be used as a really ridgid floor joist or a really bouncy diving board. It depends entirely on which dimension is cubed.
Can you tell us if you've already purchased the tub and/or the plastic tub surround?
Also, can you tell us what the tub is made of? (ie. acrylic, cast iron, enameled steel)