You have to know in advance to not allocate 100% to root and home otherwise you are SOL when you want to make space later. If you're lucky you can disable swap and temporarily use its allocation to do it, providing that is large enough for the changes.

This is not the case, as like I said you can shrink either of those filesystems and its container and use the freed space for this.

(Also I think lvm doesn't need the volume blocks to be contiguous on the physical volume. So you might have N free space after volume a and M after volume b, and lvm would let you create a new N+M sized volume.)

So when I have root and home mounted because I am using the computer can I shrink them? No because they are mounted.

The easiest way to resize the root partition is probably to boot from eg Ubuntu live/install USB stick. The home partition you can unmount (but not while non root user sessions are using it).

> reboot into another OS

As I said: garbage for snapshots

You need to do it only once (if you didn't do it at install time), not every snapshot.