Sorry for the late reply, I've been working so much I haven't been on here very often.
1) I would say a safe bet would be one size, I typically wear a 33 or 34 waist, and the 34 can accommodate IWB carry comfortably, a 36 is even more comfortable, but a bit too big if I'm not carrying, that being said, since I've been in AZ, I wear the Wrangler ATG pants and shorts almost exclusively and they have some stretch in the waist, so I could get by with a smaller size if I wanted to.
2) I rarely (if ever really) tuck in my shirt, that being said, I do wear a "wife beater" type shirt often, and haven't had any issues with it coming untucked.
3) Sitting while carrying in the appendix position will likely never be 100% comfortable, however, this particular holster is about as comfortable as I can imagine a holster would be, due to the shape, and believe it or not, the extended length. I don't sit much while carrying, but I've been in the car for a couple hours at a time with no issues, I've even fallen asleep on the couch while carrying (long day at work lol). I plan to drive out to CA next month, which will be about a 7 hour trip from here, so I'll give an update on that, as it will likely be the longest time I've carried in the seated position with this holster.
If you have any other questions you can think of, just let me know.