I can only share my experience regarding OBC but the short answer is that OBC is forfeited. When our Millennium Hong Kong to Shanghai cruise was cancelled we were reimbursed every penny of our cash outlay for the cruise but the stockpile of OBC that we accrued through intermittent cruise fare reductions under the best price guarantee were not transferrable to our rebooked cruise on Apex out of Southampton (which is looking more and more doubtful). I am beginning to develop a complex as my cruise selections get foiled by that virus!
Every "complaint" on these boards has to be taken with a grain of salt. I can't tell you how many times I have read "complaints" about a cruise that we too were on (which we thoroughly enjoyed) and can only assume that some complainers actually live for the moment that they can complain about something. We've had our share of different cruise lines and enjoyed each and every one of them. Were there things that we were not fond of on each? Yes but the variety of products is very appealing as it provides opportunities to be pleasantly surprised. Each brand develops its own sense of what a cruise experience should be but until you actually try them how would you know which would actually appeal to you? Enjoy you cruise (I know you will!)