Are these sentences correct

AnonymousA. He must have been driving for an hour.

The evidence leads to that inescapable conclusion. It is an action with duration in the past.

AnonymousB. He must be participating in the match on Friday. Is it a correct sentence?

Yes.
Again, you have solid evidence for your assertion.

You mean the first sentence focuses on duration and it doesn't say whether he is driving now or not, right?

Those words alone do not say that he is driving now. Indeed, in the sentence below, we know that his is not driving now:

He must have been driving for an hour when his car went over the cliff.

That's a past sentence. Can't we use the sentence with present meaning?

Yes, we can.

Can I say, 'he must have been driving for an hour. The car is hot.'

Yes, but it does not mean he is still driving. His car might have overheated and broken down. It still can be quite hot.
Cars usually heat up after only a few minutes of driving, though.

The narrative will tell you if he is still driving, eg.

He must have been driving for an hour when he passed through Las Vegas on his way to Los Angeles. He's probably halfway to Barstow by n

Nice explanations!