Solved

Tyres

I think no refuelling favours fewer stints especially at circuits where the time lost in a pit stop is large. Spain has one of the longest pit losses, the information I have suggests it's about 25 seconds.

Spain 100% is 62 laps, as a very rough calculation a two stopper is going to do approx. 21 laps per stint and a three stopper approx 16. So the two stopper is going to have to do 15 laps on tyres that are more worn than a three stopper (the final 5 laps on each of the three stints). In order for the three stopper to win he has to make up the 25 second pit loss in those 15 laps, so a bit over one and a half seconds a lap faster.

BTW, the tyre wear calculation doesn't work the way you suggest. If you get 5% tyre wear reported in practice it doesn't mean they wear to 0% after 20 laps.

Pit loss time.
You can get a very good estimation using a couple of accounts in a private lobby quick race. By managing the cars correctly you can avoid DRS or Boost effects from skewing the results.
Alternatively you can use your post race data but doing it this way it's difficult to eliminate DRS influence.

In some races, yes.

Last season in our 20 race league, Elite tier, Hard tyres were used in the winning strategy in 5 races. So that's a quarter of all races featured Hards in the winning strategy.