1) To be honest, I'm not sure how many hosts can connect at once. I know it's at least 3. If you'd like to use sprint hotspot, you can flash a ROM that has it unlocked. An example is myn's warm or mikfroyo. There are several of them.
2) Gingerbread is Android 2.3, and there are a few beta ROMs out there, yes.
3) You don't need to be rooted at all to change the launcher and whatnot. You can use Launcherpro or ADW, for example, to change the UI a lot. If you'd like to keep sense, but change the theme or the way the launcher looks, you can flash a custom ROM or theme. You must have a deodexed ROM to flash a theme, though. Almost all custom ROMs have this ability, though.
Here's a list of all custom ROMs:
HTC Supersonic/ROMs - XDA-Developers
You can see in the fourth column what the ROM is based off. If you want keep sense, flash a ROM that is based on sense. You can tell which are sense based on whether or not the base is an official OTA number. 3.70.651.1, for example, is the latest OTA. Anything that says AOSP is built off Google source code, and does not include Sense at all. These types of ROMs do not yet have working 4g or HDMI. 4g will be working soon, though. It's in alpha, right now. Down at the bottom, you can see there are a few ROMs built off AOSP 2.3. Those are the gingerbread ROMs, and they will be buggy.