I'm assuming you're asking about how to remove the freewheel, as the Mongoose Rebel 20 is a single-gear BMX. In that case you need the correct freewheel puller to get the freewheel off. Check out www.parktools.com for more info.
I don't really know what you're calling a driver, but it's probably the sprocket and the freewheel mechanism. And yes, it can be removed. You need a special tool, a freewheel puller, and it has to match the make of the freewheel that you have. check out www.parktool.com for more info.
There are two different hub designs for BMX, freewheels and freehubs. You need to know what you have if you want to replace the sprocket by the rear wheel.
you would need a cassette. it would have to be 10 tooth
They're threaded in place. You need a special freewheel puller (park tool sells them, among others.) To remove the old one(which is reverse threaded) and it can be torqued beyond belief. The new one spins on by pedalling action, so no need to torque that one.
It's not really a question of better, more of different. What decides how fast the top speed of the bike is, and how nippy it is off the start is the ratio between the tooth count of the crankset, and the tooth count of the freewheel. With a smaller freewheel you can have a smaller crankset, a slightly lighter bike and get the same feel of the bike as with a bigger crankset and a bigger freewheel.
They're threaded in place. You need a special freewheel puller (park tool sells them, among others.) to remove one and it can be torqued beyond belief. They spin on by pedalling action, so no need to torque that one. There's a newer freewheel design that sort of mimics the design of MTBs, with a freehub body and a separate sprocket. they need an different approach.
Unless by freewheel you mean driver (in which case 8-9) I haven't ridden bmx in a few years but last time I checked the smallest freewheel you could buy was 12 tooth which would make for quite a low ratio
possibly, if you have the kind of hub where its solid and you can put on a single piece freewheel than no. But if you have the kinde where you need the single piece freewheel and a bolt to hold it on that yes. what you do is spin the thing that holds it on off and screw your nine piece driver it.
Of course it will, but in my opinion freewheels arent the best
If you have a flip-flop BMX hub, you can put a 14T on the smaller diameter side of the hub. If you have a cassette hub, you can get as small as 11T.
It's probably a busted, worn or seized freewheel. They can be serviced, but it's usually not worth the hassle as they're fairly inexpensive. As you need a special tool to get the old one off, better take the bike to a shop and just have them replace it.