While reading more about this I found out about the ~/.hidden file which I'm pretty excited about. The following hides the folder from Thunar & Dolphin (and presumably other file managers):

echo snap >> ~/.hidden

But sadly `ls ~` will still show it.

Question is if those user/mount namespace tricks are composable

As far as I'm aware, yes! You should be able to nest namespaces and mounts.

I was wondering the same. So infuriatingly obnoxious!