My Character has no way of getting the legendary Cloak because I accidently abandoned "An Unwelcome Visitor"

Might not help but there’s a few work around you might be able to try in the bug report forum

https://us.forums.blizzard.com/en/wow/t/an-unwelcome-advisor/681695

For some abandoning the quest “On whispered winds” put Anduin back, for some they used Party sync to phase the keep.