Stability in the sense of: my computer does the thing i expect with the hardware i happen to have, every time, over many years.
I agree Debian is up there. I only mentioned Arch because of the massive userbase. I think Debian is a little more technical (for a new user with limited time and attention) than Ubuntu or Fedora, but much less so than Arch
Debian, Fedora, Ubuntu, and Arch are undoubtedly the big 4 Linux distros in terms of long term community, stability, and documentation
There are probably boards that come with them but it's not typical. You can buy a pack of 250 for probably $10 online, they are easy to come by.
you put one o-ring around every key stem. The o-ring hits the edge of the switch after it actuates but before the key bottoms out, so you hear rubber on plastic rather than plastic on plastic
I saw some images online with people using two o-rings, but i'm not sure if there is any point to doing so. Perhaps for very tall key caps.