Can't help you with 1. or 2. since I haven't tried any 2.1 ROMs. However, the SMUpdater you bought/ran saved a nandroid backup so if you play with 2.1 ROMs and they turn out to be crap, you can always load a backup. In fact I recomment making an Advanced Nandroid Backup every time you try a new ROM so you can go back to immediately before that.
3. You are rooted and have the recovery. All you need to do to flash a ROM is one step essentially. If the rom comes in an update.zip then you put it on your sdcard, boot into recovery, go to install > allow update.zip, then install update.zip. Done. If the ROM comes as a Nandroid backup then you put it in sdcard/Nandroid/ and then boot into recovery and go to Advanced Nandroid Restore or whatever it is. Pretty easy.
4. Mine lets me get up to 1000 with setCPU, not sure why you can't...
BTW since you have the recovery image flashed already and a backup, it is VERY hard to brick your phone
