Sounds like you are doing great up there. Consider yourself very lucky to have a second flush. Maybe even the inflorescence will turn in to a branch.

When and how would be a good time to prepare them for bringing them in for the winter?
When the temp dips to 50 you can move the pots inside. Gradually move the plant to their winter spot inside your house.

Should I choose a minimum temp to start bringing them in at night? right now sounds like perfect. But you can probably give it a couple more weeks of outside time.

Should I plan on letting them go dormant and cold store them or bring them inside where they will get some of the light for my seedlings (near a east facing window)? Plumerias like to go dormant. Inside your house they will be very happy. Put them by the window and water them sparingly, once a month if needed.

I've cut back on fertilizing all of my plants - Is this okay for the plumeria or should I continue to fertilize them? Yes, stop fertilizing during dormancy, Plumies only need to that during growing and flowering season.

Any special techniques I should be aware of when I un-pot them (to check for slugs)? Plumerias have very delicate roots. It would be great if you can let them stay in their current container. You can check them next year.

First frost date is usually about a month away although it's been a cool summer (except for a few days here and there) and it's been getting colder at nights than normal (here around 55 --- some places in the NW have already had frost warnings) Definitely start moving them inside.

Good luck to you
LL