1. It should be able to handle that threshold, I've got similar ones in my bathrooms and it gets over them without issue.
2. I haven't experimented with multiple floors, but it shouldn't be an issue. The in-app FAQ says it'll create a different map per floor, so you'd just select that map to start it on a different floor. I'd imagine it'd notify you when done or full and you'd have to return it to the base station. It also has edge detection and doesn't fall down stairs.
3. Unfortunately, I inadvertently updated my firmware before digging into Valetudo where they recommend not updating. I was going to join their telegram to see if it's still possible. The site says rooting is straightforward, but their telegram would probably be the best place to get assistance.

I was able to flash my Dreame D9 pretty easily. I had to use little wires off of a usb dongle which I got both off of Amazon. It looks like they sell a dongle on the Valetudo site specifically for Dreame devices that might be worth it in your case. https://github.com/Hypfer/valetudo-dreameadapter

I remember coming across a thread somewhere about creating an automaton to replace map files for different floors in your house but I can’t seem to come across it again. You could map each floor, export the file, then restore floor map files as you move the vacuum to other floors.

I just went through this process when I got a Dreame D10 Plus which has an identical root process as the L10s a few weeks ago with the intent of putting Valetudo on it. I ended up trying but failing after one of the header pins I was trying to flash it with broke off in the machine. If you are brand new to this stuff, personally I wouldn't recommend trying it. If you still want to try, do not update the robot once you get it and either buy one of the programming boards or at the very least have some 2mm pitch header pins around (standard in NA is 2.54mm). I really wanted to run mine locally as well (don't like the idea of someone gaining access to a full lidar map and remote controlled camera in my house) and hope in the future an easier method is discovered. I don't know about the base station for each floor thing, I do know that Valetudo only supports a single map. From what I understand that means you'll have to re-map each floor before cleaning. If you only want Home Assistant intergration there is a HACS intergration for Dreame that allows for multiple maps. You used to have to jump thorough some hoops and use it with Xaomi cloud or something but the newest beta allows sign-in using the Dreame app credentials.