In summer with light clothes, it is hard not to print at all when you bend over, etc. Is it in your law you can't print? Or show?
Here we can open carry. Printing isn't an issue. I carry a G29 or G27, both in a Milt Sparks VMII holster, and I can reasonably conceal them with an Underarmor polo shirt and can conceal them well with an untucked unbuttoned Tommy Bahama Hawaiian shirt. If the wind blows and it shows or if I bend or move such that it prints, I don't care. I don't worry about it. Also, it's amazing how many people don't notice it when the wind does blow... And most of the ones that do notice it don't pay it much mind. Still, I make a good effort to hide it, I don't want to advertise, but I'm not to concerned about it showing if it does. In some places though, this is a no-no.
For those places, you may have to get something thinner, like a small Kahr, and carry it deep so only the grip sticks out. That or pocket carry. The belly bands look like they could work, provided the band doesn't curl. You can also get ingenious and design your own belly band and make it. They are easy to make, and you can make a better one than the ones they sell. If you can't print at all, and you want to carry IWB, most likely you are going to have to give up that M&P. The stainless Kahrs would be my choice most likely, stuffed in a Milt Sparks holster. Oh yeah, while I'm mentioning holsters, get a good one made by a good maker, not some mass produced junk. You'll be glad you did and you will save money over the long haul. Also get a good gun belt. It doesn't matter how thin the pistol is and how nice the holster is, if you have a cheap dept. store belt, it won't work as well. A good heavy thick gun belt holds it close to your body without it folding. Makes a world of difference.
As far as rust, well, you either need to get something that doesn't rust or something that you can carry and not worry too much about. I thought the M&P's were stainless? In that case, I wouldn't worry too much about it. I carry Glocks and they aren't stainless, but tennifer treated and coated, and I don't have any problems. They will show wear from going in and out of the holster, and they will get bumped on things like door frames and such over time. Just keep it clean and that is about the best you can do. Your carry piece is a tool, not a showpiece.