I think there is a good chance you will need to replace the valve inside the wall. If the tub/shower is tiled, can you get into the wall from the backside? i'm in the same boat, and finding we cannot replace our trim without changing the valve as well.

We use Moen valves and faucets. A professional, licensed plumber, experienced in performing a tub/shower valve replacements can cut the tile with a grinder or multi-tool, install a new Moen valve and install a Moen repair plate to cover the holes from the previous two handle valve. If performed correctly, there will be no evidence that there was a two handle valve except for the repair plate. It is recommended that you have a licensed plumber perform this type of work, as it will require soldering in the wall in a confined area. This is not something the average DIYer should attempt. Typical cost $800-$1200. Trust me, it's worth it. We have to bail DIYer's out all the time who try projects like this and get in way over their heads. Do not go with the cheapest tub and shower faucet. It is not an easy job to replace one, once it is installed. Purchase the best that you can afford. You only want to do this once. Ask your plumber for recommendations. Purchase it through your plumber, if possible, he will usually stand behind it if there is a problem. He will not warranty one that you provide. If you are going to remove the tile and tub, then the valve replacement is much simpler, however, you are turning this into a major project. Pulling the tub usually results in drain issues and flooring issues. Most of the time, this requires floor repairs or replacement and drain replacements. In most areas of the country, there are also ordinances requiring this work to be performed by a licensed plumbing contractor with a permit. If you hire a contractor, please make sure they have a plumbing license and will procure all necessary permits and inspections that your local municipality requires. If they will not do this, look elsewhere, no matter how cheap they are. Good Luck with your project. Here is a link that might be useful: Lemay Plumbing, Inc., a St. Louis Plumbing Contractor

Who will be using that bathroom? My oldest took baths exclusively until he was six. My youngest is still taking only baths at five. So ideally, I'd have a tub in bathrooms used by kids. I know you said your kids are gone, but do you have visitors with kids or grandkids now or potentially soon? For resale, if I had kids and was looking to buy I'd prefer a house with a tub in the bathroom used by the kids, rather than storing their toys in my bathroom and having them troup into my bathroom. Same with a guest bathroom, it would be nicer to have the ability to bath my kids and I would definitely not want to impose on my host to use the master bathroom. On the other hand, if the bathroom will be used primarily by elderly visitors, a walk in shower would probably be better. This is very much guessing, but I'd think cost is probably a wash? The tub will be more expensive in materials but cheaper in labor. If you go with the inexpensive kohler cast iron tub available at Home Depot, you are probably ahead of a tiled shower floor in cost? The tub is definitely cheaper than a cast iron shower base.

I think sometimes you just have to work with what you've got. Green marble (and this looks like cultured or acrylic), is a tough thing to work with, but I think going all white with the rest just makes it stand out more. Would you be open for wallpaper?