If you want immutability, Vanilla is the only good option right now. Services can be a nightmare on Fedora’s immutable systems, and some applications (qbittorrent, in my experience, though I haven’t seen anyone else have issues with it specifically) sometimes just outright decide to off themselves. I wouldn’t say it’s a bad distro, but recommending it to someone who states they don’t know much about computers could cause them trouble in the long run.
If you’re looking for something that won’t break, Debian and openSUSE Tumbleweed are two good options. Both offer the Plasma desktop, though openSUSE may have an easier installation interface for some. Note that some video codecs aren’t (officially) available, so if playing “differently acquired” media is a concern, perhaps Debian would be easier.
If you wanna try arch, consider EndeavourOS. It simplifies the installation process significantly, though it doesn’t do much to help maintain the system. That’s on you. Avoid Manjaro like a plague.
2
The Arch Wiki is universally considered the best source. 99% of what you’ll see on there will work on any linux distro, so don’t worry about the name. Aside from that, your favourite search engine is your best bet.
3
No clue what FALGCS means, but wallhaven.cc a great place for wallpapers.
Edit: seems the Manjarno site is down. Shame, genuinely useful site for times like this.