celcom will throtle your speed after exceeding 5gig bandwidth usage every month.. so, if you are using 5gig on the first day, then you will experiencing very slow connection (or sometime cannot connect at all) for the next 29 days until next month.. so, please use wisely 

but so far, celcom not so strict about that, that is why some user still can surf/download as usual even after they exceed the 5gig limit.. but celcom technical department now are about to start to throtle your speed. so, beware soon!

5GB bandwidth inclusive ingoing and outgoing bandwidth, but this is based on your luck. You may or may not being capped. Some user already reach 100GB without getting capped

It is very subjective, my speed got capped twice. Once on D68 and once on D98. But *touchwood* it was 2 months plus since my last cap, and I have downloaded more than 50GB in these 2 months. Still hanging on there.

So, you may have or may have not the chance to get capped. Many factors involved, total users in your area and luck.

Yes, kind of. That is what you should do.
Already verified if your place has HSDPA?

There is no online registration for Celcom BB as far as I'm concern.
Just go into any Celcom branch, and you are off to good. But, I'll suggest you to go straight to Celcom centre as it is faster to get your connection activated. Nevertheless, my hometown has no Celcom centre and I just went into one of Celcom branches and did my registration there, voila it was activated within 4 hours!