Dagoth Ur says my PC is unprepared

Wraithguard, Keening, and Sunder are all in my PC's inventory but Dagoth Ur says I am unprepared -- which suggests that not all of the tools are present per the Unofficial Elder Scrolls Pages entry on Dagoth Ur.

I am going to try leaving his stronghold and returning the next (game) day to see if that makes a difference.

Any other suggestions?

I figured out how to complete

I figured out how to complete the quest despite being "unprepared." Did a web search and found a comment posted somewhere that said to load up on Fortify Health potions before following Dagoth Ur into the next chamber. Then I ran right past him to the Heart of Lorkan and did what had to be done.

Switching between Keening and Sunder caused a Mortal Wound spell to be inflicted on my PC. So I stayed with my PC's standard weapons until it came time to smash the Heart.

Lore states that Wraithguard is only a single gauntlet. Is it possible to get the other one somehow without killing Vivec and taking it?

There's only one Wraithguard,

There's only one Wraithguard, but because Oblivion doesn't allow separate gloves, it is displayed on the character as a pair of gloves and not a single glove like it's supposed to.

You only get "Mortal Wound" if you equip Keening\Sunder without Wraithguard equipped on your character.