1 – You get a removable sealed valve (at least with xm819’s) supplied with the rim/wheel. Has a rubber lump (square/rectangle) which sits in the well of the rim as you push the valve through, then a rubber ring for outside which sits inside the thread nut thing.
2 – Err, I have on occasion been known to use levers when trying to fit non-tubeless ice spikers (see answer 4), however all the UST tyres have gone on with soapy water, thumbs and I generally use a track pump rather than a hand pump. I don’t own a compressor.
3 – No idea, I’ve only used UST tyres with UST rims.
4 – Easy – remove the valves from the rim, fit tube and tyres as usual, I’ve not had problems with any except ice spikers which are a grade A “baby fish”. Be gentle removing the valve to protect the rubber ring and thread everything back together to prevent loss of individual parts.
I’ve given up on using sealent too – I’ve had 1 tyre with a slow that required it but nothing else in 4 1/2 years of cycling YMMV.