In my old apartment we had 9 foot ceilings and single uppers that went all the way up--the top of the doors was maybe within 4 inches of the ceiling. The cabinets must have been nearly 5 feet tall. Yes, we used a step-stool to get at the stuff on the top shelves. We kept infrequently used things up there--the electric frying pan, some nice serving dishes that tended to only come out at holidays, things like that. I remember once reading guidelines about what the maximum cabinet height should be and ours were well beyond that. But we never had any issues, even though they were ancient (not sure if they were original to the 100-year-old house, but if they were installed after the 30's I would be very surprised), and had inset doors. Modern hinges should only improve the performance.

Can't, sorry, I live on the other side of the country now. I did have 9' ceilings. If you are worried about how it will look IRL, do a mockup. Get a large roll of butcher paper (the brown stuff), and tape it to a wall. You can do it outside if you can't find a workshop or some other high space. Then draw the doors, a window, base, and crown. Better still is to have a carpenter do a mockup, with the real millwork, on a sample wall. The wall does not have to be more than a few feet wide. I would only do this in an expensive house, obviously. In fact, if you are getting this fancy, you probably should have a designer, and that person can supervise the mockup. IMO this is the only way for a layman to get a good idea of how the elements will relate in reality. My design was a "modernized" version of a colonial house, almost Shaker, with flat stock mouldings and a flat crown. That's probably not what you are thinking about, so it would not help you. Good luck.

Check with your builder to see what the difference is between 8', 9', and 10'. To stay proportional the windows and doors may need to be custom sized to fit. We had a 9' ceiling for 25 years. We just moved to an 8' ceiling house. I'm 6' and don't have a problem with the difference. With 9 or higher you will need a ladder to reach higher. With 8-ft I use a step stool. Heat does rise. In the winter this means you will want a fan to stir the warm air down with the cooler air. I used a table fan placed on the ground and aimed toward the ceiling. It makes a big difference. In the summer the same thing happens. I run the fan 24/7/365. Smoke rises, too. We had a house fire several years ago in the 9-ft house. It started in the kitchen where there were no smoke detectors. Smoke had to fill the kitchen ceiling down to the door frames before it poured into the dining room. Then it had to fill the dining room down to the door frames. Once it filled the dining room it had to fill the living room down to the door frames. Once it filled the living room it set off the smoke detector in the hallway. The fire burned for at least 30 minutes, and I was awake the entire time in a lower part of the house. So if you are wiring for wired smoke detectors, keep that in mind.

If I ever build a new house, a dedicated Christmas tree closet would be on my must-have lists. It would be on the same floor that the tree is on and big enough so that the tree could easily fit in there fully assembled. I would leave the tree fully decorated, ornaments still attached (I attach my ornaments with the wire hangers wound around the branches a couple loops rather than just hung there so they are secured and can’t easily fall off when bumped/knocked), and the whole thing would be on wheels so I would just have to wheel the tree out of the closet, plug it in and voila! My current tree is 9 feet and while I do enjoy decorating trees, it’s not AS enjoyable when no one wants to help or cares that much about it. I grew up decorating the tree as a family so it felt like a tradition with family bonding time rather than a chore/task like it does now. A large tree can be an undertaking when you’re stuck doing it by yourself, especially if you’re like me and like a full tree! I do get help when I ask but it’s much more enjoyable knowing others enjoy it and look forward to doing it without me having to ask.