Can one reuse positive referee reports if paper ends up being rejected?
For many journals the referee is asked to tick a box when they submit their report to indicate whether or not they (1) allow the report to be used for another journal and (2) whether their identity may be disclosed to the editors of that other journal.
Given this practice, the answer to question 1 would be a "yes". Whether or not the report is then actually transferred from journal X to journal Y (anonymously or with the identity of the referee disclosed) would be something journal X decides based on whether or not the referee authorized them to do so.
Concerning question 2, I don't think you need permission beforehand from journal X when you inform journal Y of the existence of a detailed report.
This complication will hopefully become a thing of the past, when more and more journals migrate to a practice where referee reports are public documents, even if anonymous.
This is not really an answer, but it might be informative. The BE Journals in economics (of which there are several: The BE Journal of Macroeconomics, the BE Journal of Economic Analysis and Policy, the BE Journal of Theoretical Economics, etc) label each published paper as belonging to one of four "tiers": a Frontier, an Advance, a Contribution, or a Topic. Frontiers are supposed to be papers that would be suitable for a very high-end journal, Advances suitable for something a little less selective, and so on.
When you submit a paper, it's considered simultaneously for all four tiers. The idea is that instead of aiming high and then re-submitting a little lower down after a rejection, one effectively submits to four different journals simultaneously (and there is a single refereeing process where referees recommend not just acceptance, but acceptance to a particular tier). I'm not sure how successful this has been, but it's been successful enough to survive for, if my memory is correct, about 25 years now.
The following might be useful to have in mind:
if your paper is rejected at journal J1 and submitted at J2, the information at the J2 editorial board that it was rejected at J1 might be considered as negative, especially if the editorial board of J2 considers J1 as being of equal or lower level (which might not coincide with your own appreciation, or of J1's editorial board or referee in case they recommended J2 as alternative — keep in mind that the appreciation of the journal level varies according to subtopics and countries).