Ameriwall has a tub surround with a 72" back wall. It can be cut down for a smaller tub. Might be others, that's the first one I found. 7230TA

Use greenboard, and do the transition behind the swanstone, just before the edge.

My acrylic Bain Ultra air tub was set in a bed of mud with a piece of think plastic between the mud and tub. This was done for the ease of removing the tub if ever the need arises. It's as solid feeling on the bottom as my husband's Kohler cast iron tub. As for cleaning, that's a different story. You're not supposed to use anything abrasive on acrylic. Believe it or not, Scrubbing Bubbles is considered abrasive :( That's been my go to tub cleaner for years. To clean my acrylic tub RIGHT, I use Gel Gloss. It's a pain (literally!). You spray it on, let it sit, buff it and let it dry and then wipe the residue. When I'm in a pinch for time or physically limited, I will use Scrubbing Bubbles. There are very few non abrasive cleaners out there. One Bain Ultra suggested was Lemon scent Lysol. NOTHING works better than Scrubbing Bubbles!

I don't believe acrylic shower surround is a proper substrate for tile. Tile needs to be set on a structurally sound platform or you will end up with cracks. Additionally I wouldn't be confident that the thinset could adhere properly to the acrylic. Your biggest cost is going to be the construction and tile work. Do it correctly or you will be disappointed with a failing shower down the road. A new tub is really not that expensive in the scheme of a bathroom remodel: I see several on home depot for under $200. If the contractor is suggesting cutting the acrylic, I would be concerned whether he is doing proper construction and waterproofing. I am not saying he isn't, but that raises red flags to me and would make me concerned.