No, you can just cut your chain with a chain breaker or just take it to a shop and they will do it for you.
they can range from 46teeth on a chain wheel all the way down yo a 22tooth chain wheel.
No, a 40t is bigger. 40t vs 20t refers to the number of teeth(that engages with the chain) that each wheel has at its circumference. The tooth size is fixed for all bikes that use the same chain spacing, so a 40t chainwheel will have double the circumference of a 20t wheel.
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.
they keep your chain tight
Undo wheel nuts, slacken chain tensioner(if applicable). Push wheel forward in the dropouts. This should give enough slack to unhook the chain. If you want the chain completely off you probably need a chain breaker tool. Some bikes are fitted with quick links/master links that can be opened w/o tools. Just look for a link that's different from the others.
it turns the wheel
To revert or ride backwards you need to pedal backwards with the chain and the speed of the wheel and then when you get to the right time you can twist around either doing a bronco or fakie or a wheelie around commonly known as a half cab.
Two, but this is not the same as how many speeds the BMX bike in question has. A BMX bike has a sprocket in the front, and a cog in the back and each is a gear, but is considered a single speed bicycle because two gears and a chain are required for transmission of rotational force from the crankset to the rear wheel.
You might have to either lengthen or shorten the chain, depending on if you're getting a bigger or smaller sprocket.
inox or chain lube from a bike shop
Five. One for each wheel, one for the cranks and one for each pedal.
No. A wheel that's 2" bigger in radius won't clear the fork crown, and probably not the chain stays and the brake bridge either.