I'm glad he's doing OK
. As for glass tanks, none of my current 5-gallons are glass...they're all acrylic, which is very light and easy to lift. A 5-gallon wouldn't need full water changes, though, so you wouldn't be doing any lifting. See, bigger tanks can be cycled easily (I made a sticky a while back in this section) which means you'd only be doing partial water changes (scooping out some water and replacing it...maybe occasionally gravel vacuuming unless you get good plant growth).
Don't move him to a 1-gallon, either, just do a water change on the 3-gallon and move him there. If there was anything toxic in the 3-gallon, it's probably gone by now and if not, will be removed when you change the water. Don't bother changing the gravel, either, any beneficial bacteria in the old gravel are dead so just keep the new stuff
I'm definitely thinking those shrimp were the brackish ones. They can survive in freshwater for a bit (like at the store) but not for long. Some can be acclimated to freshwater but it has to be done slowly. Look into cherry shrimp, there's no guessing with those. You can even get them on eBay, haha. That's where I got my first batch. They love live plants, especially those marimo balls you have.
Keep the plants and don't worry about rinsing them. Ammonia is easily removed by just changing the water in the tank. By now the plants probably absorbed it, anyway
. So you should hopefully get a reading of 0 with your water tests. If not, we'll figure it out!
The water sample could be collected tonight, yeah, just store at room temperature. As for the shrimp, that kind of depends on your store's policy but I would put them in a bag and store them in the freezer.
That sucks about your local pet store, by the way
. I get almost everything online but I know shipping to Canada is insane sometimes (and I ship stuff to Canada and some online prices are NOT justified!).