Did you even read that article I linked... it pretty much tells you everything...
*Sighs and pulls your shirt collar back and tosses in tainted Anacharis*
Anyway... Apple/Mystery snails are VERY resilient creatures. He/She should regrow the missing part within a few weeks... they can regrow an eye stalk within a month. It won't be as pretty, or as long but they'll be fine.
A snail isn't the smartest creature but they are not by any means stupid. Since your Betta attacked... it will start to wise up to your Betta and hold most of it's appendages like stalks under the hood of it's shell if it senses your Betta nearby and start to react quicker if under attack. This is normal and once your snail starts to react quicker... more than likely your Betta will lose interest and ignore the snail all together.
AS for your betta's scales... eh, Drax has a small patch missing on his upper forehead along the spine, not to be confused with the pink diamond on his head (Thats just due to his Marbly-ness) and it doesn't seem to bother him... sorta like how his eye has never slowed him down. I'm not an expert in scales but I'm sure fish lose scales all the time and I'm SURE they heal over time. Thats like saying when you scrape your arm... you never grow skin back.
Last thing... READ THE BLOODY ARTICLE I LINKED BEFORE! After I read it I feel VERY confident in selecting a few snails which I shall name Lucy and Desi! I'm not mad just... the article is so interesting to read for some reason... like they even mention you can eat Apple Snails like Escargot (Apparently they have a similar flavor). I wouldn't worry about washing the snail off...
Snails are more in common with Parasites than fish. So never have them in the tank when treating your Betta! As well as... Snails are always blamed for parasites but Apple Snails in particular are VERY good at not transferring diseases, and if you got the snail at a petstore (Yes even a Walmart one) you can be 100% (Or almost) sure your snail isn't carrying anything nasty. The only nasty thing they can have is the Rat Lung Fluke (Or something) that won't hurt your fish but can ACTUALLY kill YOU but as I said that's only if the Snail was living in a sewer with Rats... and you could only get it if you ATE the snail.
Bottomline... Oh your getting as paranoid as ME! How cute! Be sure to tell me about your first mental breakdown! Just put the snail in, it can hold it's own.
As always... if it goes terribly wrong... blame me haha.
Janus: Double Tail
5 Neo Tetras Larry, Curly, Mary, Tyler, and Moore
R.I.P. SOTTO: Medium CT Teal
R.I.P. Drax: Marble Male VT