Some collocations, especially adverbs, are quite idiomatically 'set', hence better to learn the more common usage. I seriously doubt you would score lower than a 7 if you use language like this correctly in IELTS!!

Hi Aliona, I wouldn't say it's a collocation but they are both commonly used phrases. Perhaps highly doubt is slightly more formal. One thing I would say is you can say 'I highly doubt he will...' thereby taking out the 'whether'. The whether is not necessary here. Hope this helps!

"highly doubt" is correct, but "whether" feels wrong to me; I would prefer "that", so the sentence would read: "I highly doubt that he will actually carry out his threats". "That" can be omitted, leaving "I highly doubt he will actually carry out his threats".

At least in American English, there is not much of a difference at all between highly doubt and seriously doubt. Who told you that it was incorrect? I agree with Catherine that whether is unnecessary here.