If you'd bought a 2TB you could just dd image the windows disk to the new drive. If you can convince Windows to downsize its partition and then use a partition editor on a USB liveboot to identify the drive sectors you could maybe still image the windows disk. Big "if."

As to the second question, use block IDs-- the filesystem's UUID. Grub is lazy and assumes a single root (the first found) partition so if you want a particular boot entry to use a specific root slice, you'll need to ensure each OS entry in grub uses the right UUID for its kernel root parameter. Loading the right root gets you the rigjt /etc/fstab to mount that root's expected partitions

Honestly you'd be better off running your stuff in VMs. Dual-boot is a nightmare.

That shouldn't be a problem. You can even install them on the same btrfs partition if you wanted to as long as each distro gets its own set of subvolumes for stuff. Separate partitions and even separate physical disks? No issues there, that's even less weird.

Ideally what I'd do in this scenario is at least make a subvolume for the Steam library that way you don't have to mount the actual home folder, just the Steam library subvolume. I also have a separate subvolume for movies and TV shows, and a few other things. It's just very convenient for organization purposes, and also technical purposes because now my home snapshots don't take all that much space as all the big data stuff is separate. No point backing up a Steam library.

But in the end, none of this really matters, you can mount anything just about anywhere. We all already mount a FAT32 at /boot or somewhere similar because UEFI requires it. The filesystems all have UUIDs which are usually used for configuring fstab and GRUB and whatnot, precisely so even if you physically swap the disks or even put it into another computer, it still works.

I usually use a gparted live image so I have a visual representation of what I'm working on, and none of the partitions are in use. But gparted is getting pretty old and has limited support for newer filesystems and features.