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If Deathmarks reembark in a Night Scythe can they pick a new unit to target for Hunters from Hyperspace when disembarking the next turn?

No

As above. They disembark, not deploy.
You also, unless I've missed something, have absolutely no way to embark a nightscythe past deployment; the invasion beams rule only allows them to disembark, and as the flyer can only zoom (no hover) you have no way ot then embark.

Even if you were able to re-embark, which you are not, the FAQ clearly says that each unit of Deathmarks may only mark a single enemy unit during the course of a game.