Im pretty sure that everything below 160-150 should be mono, but you could experiment i guess. Regarding the high frequencies, a lot of people tend to complement their kicks by adding hihats or little "toks", which may be in mono or stereo, it depends on your taste.

it depends on how wide you want the kick to be. if you want it to be strong and punchy then mono is fine. but if you want it to be big and on a slower tempo (>120) then maybe making it stereo would be a really good trick. try analyzing porter robinson's worlds track and check how the kick is

Depends on what other trade-offs you're willing to make. Keeping the main meat of kicks (and snares) mono gives them more perceived power and weight in the mix. It also keeps these more powerful sounds central, which keeps width and room available to other sounds. If the bulk of your kick or snare is stereo, it's more competition for other elements in the mix.

That said, the main importance of having the kick mono is to preserve solidity in the lows, and on a technical note, anything stereo under 150hz will make cutting a track to vinyl a nightmare (which is admittedly of less importance today). Definitely don't restrict yourself to making the entire kick mono though; if it has a great stereo upper range that sounds fantastic, keep it! It's mainly the low mid/low end being mono that makes a lot of difference.

definitely keep the low end (~<130) of the kick mono but the high is up to you and how it sounds in the mix depending on where the other elements of the track are placed

Pretty much only keep the kick mono.
I haven't done snare mono as others have said, but it could work.
Always comes off as a little flat to me.

The low end of the kick drum should be in mono to avoid phase cancellation. As for higher frequencies it's up to you. but if the stereo content of it doesn't change anything in the mix, make it mono

Whether you want your kick to be mono or not is up to you.  Personally, I'm not a fan of mono kicks.