≈ Sure, you can take samples from individuals from before everything went belly-up, but what do you do if the machine just pulls the poor soul through reality into the lab you're using instead of turning them into slime-DNA? What then? How do you even begin to explain such a traumatizing event so poor, sweet, little Troll Susie, who was just minding her own business and spending time with her now-long-dead lusus? ≈
≈ An interesting proposal, though that runs its own risks. What if the unwitting donor is an unpleasant prick? That kind of thing can carry through DNA, you know. ≈
A sufficiently broad sample averages out desirable and undesirable traits. It's the entire reason why the existing slurry on Alternia functions to produce viable trolls, in conjunction with the trials. But for a more hands on approach, your species already has remarkably advanced bioengineering capability even ignoring ectobiology, so use it.