Why did Zash ask for Khem to be present during the ritual?

Solution 1:

From what I've been able to gather this was an attempt by Zash to both take your body and gain access to Khem Val as her servant in doing so.

As it seems she is afraid the ritual may disrupt the bond between you and Khem, causing it not to serve her, if he is not present during the time the ritual is performed and still bound to serve your body. This would also be why she did not want andronikos present as he would not end up bound to serve her in your body and would simply flee or attack openly instead.

Before the battle aswell, she claimed that the bond between you and Khem Val was strong. The Dashade could be a useful ally.

I'm not sure if this is exactly what you were asking or not, but either way I hope it's helpful to you or anyone else following up on this article.

Solution 2:

Latter in the saga, after the big fight with Darth Zash, you find that Zash had gathered the bones of Tulak Hord and was keeping them to use against you and Khem as a kind of "backup plan" if you tried to stop the body swap or something else went wrong. She also thinks she can use them to defeat or control Khem if he tries stop the ritual. Max out affection with Khem Val and he will spark a quest on Hoth that explains this.