The only loach that I know of that will not eat snails is khulis. I have never kept sids so I'm not certain if they will try to eat the brigs.
Sorry about the pond snails. They are really hard to get rid of once they get in your tank.
Hopefully the water changes will dilute the alum. Other than moving them temporarily, thats all I know of that you could have done.
When you move Brigs from one tank to another it tends to leave etched lines on them. It's not really a problem but makes their shells look a little odd. I asked because I can clearly see lines on the shell in the picture. Different water conditions make a line when you move them. It's odd, but I've seen it on my own snails.
You need to slowly raise your pH if you want them to live their full life span. A little bit of crushed coral will in general raise the pH and not harm any of your fish. You need to get it up to as close to 8 as possible. I accept 7.8 as high enough for my brigs but I also add a few drops of Kent's liquid calcium every few days. You can float a cuttle bone for them to graze on.
I order Kent's online. I can't find it in any of the stores around me and have wasted money on calcium products that don't actually add calcium.

You're doing good, just up the pH and dose them with calcium and stop the erosion.
HTH
