Firstly, I would avoid using Showsheen in the future if your goal is hair growth. :) Showsheen [and other silicone-based detanglers - ie, most spray detanglers, Cowboy Magic, etc] ends up coating the hair strands with silicone, which makes the hair shiny and easily detangled.
That coating also inhibits the hair's natural ability to take in moisture, and, if the hair dries out too much, can encourage the hair to break.
I've had good luck using natural oils [argan, coconut, even olive oil!] to detangle and moisturize my gelding's mane and tail. The both look so much nicer now that they are super hydrated! :)
I've never removed a tail bag due to wet weather. I've removed a tail bag during wet weather and found a wet tail, but it's never been a problem for me.
However, I have heard of tails getting wet and getting moldy in tail bags. I haven't had that experience yet [hopefully never will].
I make sure to take the bag off, un-braid the hair, remoisturize, wash it if it's super dirty, brush it, then leave it down while I work my horse. I try to do that on a weekly/bi-weekly basis.
Bagging a tail isn't a "bag it and forget it"-type situation! :)
I've never used a lycra bag, so I can't really help you with the intricacies of that one. I've always made my own braid-in bags out of cotton-y material - the bag itself is made up of 3 separate bags. I divide the hair into three even sections, braid each section, slip each section into it's own section of the tail bag, tie the bag into the base of the braids, then braid the sections together and tie them together at the end.
I've never had one of my bags catch on anything, but I have had non-braid-in bags get caught. Thankfully the damage was always minimal!!
A bag will help the tail stay cleaner, but it's also more fraught with "danger." The best way to know if it'll work out is just to try it, unfortunately!
You can just braid the mane and not use mane bags. Mane braids also need to be taken out quite often to avoid hair damage.
Hair care can get a little taxing! :)
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.