DE will become active again when it dries. Generally when you are using it, they recommend reapplying after rain and such because a lot of it gets washed away. I'm not sure if your beetles burrow or not. If not, watering several times might be able to flush enough of it out that the DE can't hurt them.
As far as the bait, from the terro website "The liquid ant bait, whether in our dropper bottle or pre-filled bait stations, does not have an expiration date. The effectiveness of the active ingredient, Borax, does not diminish over time." Given that it's designed so that the ants carry it to their colony, share it with other ants, and poison more of them, I would assume that beetles eating the dead ants could be poisoned as well. If it were me, I'd rather replace the soil that risk killing the beetles. If that is a huge chore and you can be really patient, you could let the ants decompose, and give your plants time to pull some of it out of the soil. Some plants really like boron and do well from supplementing with borax (what terro is made from).
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