1) How do I create a subfolder for my emails? I don't necessarily want everything in my Inbox.
How are you syncing your emails? Over the air? or using a cable? using wireless Sync?
If you are using a cable or Over the air using activesync (exchange server) .. you can create the subfolders in your Outlook and then they will show up on your Q.
You can get to them by going to (on ur Q) Outlook inbox - Menu- FOlders- menu- manage folders
2) Quite often, when I press a key down another letter or character will appear on the screen at the same time. Is this because I have to 'break in' the keyboard?
huh?
3) Is there any way to put a Bluetooth On/Off button on my homescreen? I haven't yet looked for a way, but if someone can recommend something I'd be grateful!
Someone created a utility to do similar thing.. if i find it i'll post it. But yea search for it.. Edit: Found it by searching..
4) Is there any way to turn off the notification for a sent text message? This drives me crazy.
Yes.. you need to do some registry modification. search for it 
Edit here you go.. Motorola Q hack