What do you call it when Tahm devours Kalista without consent?
Irony. 
"'Offer' to bind," my ass.
Irony. 
"'Offer' to bind," my ass.
What happens if Kench Devours a Kalista who is mid process of using her ult? Do they implode? Do they divide by 0?
Either Kalista succeeds when she casts her ult and pulls in Tahm, or Tahm Devours before Kalista's ult activate. It cannot happen the way you explain. There is no "mid process" Kalista ults, Tahm becomes untargetable and commandable.
That's a shame.