I think translation administrators should be allowed to unmark the page, but not to unlink it, if they can’t edit it – the difference is that unmarking simply removes the page from the translation system, while unlinking (and only unlinking) additionally removes the markup from the page. So a simple unmarking doesn’t do anything the translation admin couldn’t revert, but at the same time it does things a normal sysop couldn’t do, thus eliminating the need for having someone who has both rights. (Especially in smaller wikis, it’s not unlikely that there’s no user that has both rights.)

Special:PageTranslation currently links to unmarking if the page is broken and unlinking otherwise. Maybe it should be changed to always provide a link to unmarking, and additionally a link to unlinking if the page isn’t broken. (I would display the unlink link even if the page is protected – and only display the permission error once the user clicks it –, because the listing is already very expensive, additionally querying the protection status of all 10k+ pages on Meta doesn’t sound like a good idea.)