I'm pretty sure that Tserriednick's ability is a Specialist, and as such he was going ot have his hatsu regardless. The only other Specialist we've seen from before they had their hatsu to having it was Neferpitou, and she was already manipulating Paku... and then we go straight to her having a hatsu later on without it being shown what a Specialist must go through in order to have their ability. From the anime at least, it appears all Pitou needed was to concentrate on what she wanted... and the ability just sort of manifested. And it so in multiple ways besides just her Dr. Blythe. In fact, she should be a manipulator, given what most of her abilities do... and then suddenly there's Blythe who was her first ability, apparently. (She's an Chimera Ant and so the comparison is very shaky anyway).
For Tserriednick... we see how it develops and it seems like creating nen beasts is his Specialist power. I don't believe the time manipulation nen beast is even his only use of this power. It doesn't make sense to me. It would be more rational that he has a power very similar to his ancestor who created the jar ceremony. We discussed this way back when his ability first manifests.
I do agree that Theta's scheme was what activated this hatsu. Honestly... she should have gotten the other guard involved, and then both of them wait until Tserried was just beginning zetsu, and then attack directly with melee attacks AND pistol fire. Theta waited, if I recall, and then took aim to fire, and that was a mistake. Then again, if he was a weakling, they should have assumed they could maybe get away with assasinating the man if they were willing to chance casualties from his nen beast. How long could his beast survive after death? Seems like Kacho is an exception because of her purpose and connection with Fuugetsu, but I was assuming the beasts die right afterword and get absorbed by the jar ceremony's hidden room apparatus.
It could be though, that killing Tserriednick would have released an Revenant Nen guardian beast on the path for revenge. No doubt there'd have been carnage, given Tserried's personality. Theta and her comrade at least would have died in that case. So I guess the only answer was to aim for killing Tserriednick faster, or find or create an specific hatsu for putting him forcefully into Zetsu.
I'd have assumed honestly that with a beginner like him... it should have been possible to make such an ability. But I guess Theta and the other bodyguard lacked the capacities for that.