Yes. You can also use adverbs and say it this way: "It is likely true" "It is probably true". ("probably" means the probability is high)

Yes.

Yes.

A 'rumor' doesn't necessarily have any relation to its probability of being factual. A 'rumor' is just information/gossip that a person has heard, usually second hand, from other people...'through the grapevine,' so to speak. It may be true or it may be not; assessing it would require assessing the credibility of who you heard it from as well as their sources. The other sentences are both correct. Using 'high probability' is interesting; this implies that there is some quantitative, numerical basis that resulted in a probability being calculated. However, we do use it in a casual/non-scientific sense frequently; it then basically means 'likely.'