Rhode solved the puzzles, of course. There were not that massive undead there when she came.
Bellegar reset the puzzles and made the undead-problem worse to prevent anyone from solving them as easily. Also, Rhode might have some other help or even hints from Zandalor how to easier access the vault without all the spheres and whatnot.
Zandalor was not available for the Dragon Knight so the Knight had to do all the footwork instead being able to use the back entrance (which Bellegar probably also shut down after Rhode used it).
That Zandalor tinkering with the protection is known... he took the shield of the Demon's Grave to put up the shield above Aleroth. So it's not unlikely he tinkered even more and made it easier for Rhode to access the thing.
