A. Unless it literally attacks Ainz on it's own it isn't a real threat here
B. He wouldn't need to switch it off, and he's got plenty of mana to death him a few times(probably a couple), realize he doesn't resist either of his go-to moves, and just teleport close with Despair Aura up and decide what to do then, and then considering this is Ainz vs an opponent he knows nothing about, and his paranoia is going to be through the roof, he's not ******* around in the slightest. And the Literal first enemy Ainz ever encountered in the new world he thought could be superior to him and deathed one of them immediately anyway. And that's not even getting into him being tempted to use of timestop JUST for some fodder a dragon he knew resisted summoned. He's very spammy with both.
C. Reinhard can't swing his sword if he's dead, he can't do it in time if Ainz teleports, and he can't do it at all if he's a statue in stopped time.
Edit: like, I think your forgetting in literally every fight Ainz has been in where he didn't know beyond a shadow of a doubt he couldn't be beaten, or the enemy beyond any shadow of a doubt resisted both time stop and death manipulation, he used either or, or both. Even in some instances he knew the enemy was no threat he used death magic anyway (Hi Dragons.)