2. I'm using a laptop with AMD/NVIDIA on Wayland with KDE, this is no problem. This question is in the domain of your display server and compositor, not the base distro.
I choose the previous build from grub, and then rollback my config files from git and rebuild. Every build creates an entry on the boot menu so I can go back to any previous iteration of my setup (I do have it set to only keep the last 15 builds)
1. The installer is Calamares. It's fantastic and fully user-friendly, no need to worry about it.
2. I'll speak for KDE6 which I personally find superior to GNOME. You can have either with NixOS. I don't think there's anything specific to NixOS on multi-monitor functionality. There's a menu accessed either through a keyboard shortcut or system tray that allows any of the 5 permutations laptop/monitor. And it pops up as an OSD menu whenever you connect a monitor. I'd say the functionality is 'done'.
3. I suggest you read up on how Nix works, once you understand it you can understand NixOS better. To summarize, all programs in your system are stored in /nix/store and accessed through symlinks. You don't access /bin/bash, you access the bash executable stored in /nix/store through its symlink in your PATH. And all the symlinks are stored in a single directory. This directory is called a profile. There are multiple profiles. With NixOS, not only just executables but the system configuration data (users, passwords, mounted disks, desktop environment settings, traditionally in /etc) is also stored in profiles. This way you can have multiple 'whole system configuration's all containing different system configurations, arbitrary versions of arbitrary programs, and they don't conflict. During the boot, you can choose between different profiles. It's not limited to grub, systemd-boot also works.
1. How friendly is the installer? I can get a second hand machine in which I can afford to nuke the disk and let Nix take it all but what if I need to install it side-by-side with my existing Windows install on my main laptop where I have a custom partitioning scheme? Ubuntu's installer provides sufficient flexibility to able to re-use partitions without overwriting them, allocate whatever mount point I want, resize things etc. I have been using Linux for close to 20 years but fdisk still spooks me.
2. How easy is multi-monitor setup and switching between setups? I have a main monitor that I usually use but occasionally I also use the laptop screen as a second display and sometimes I just use the laptop screen.
3. The 1st link alludes to be able to rollback changes from grub? How? What are my recovery options if I mess up the system?