Given the option of working at waist level and working at the end of long stick, I'd vote for waist level. The only trouble I foresee is fit of the tongue and groove on the finish had been applied but that would be easy enough to test.

No contest for me; I'd do the finishing on a sawhorse or table. Better job and you can seal both sides. Second coat, if needed and sanding, if needed. No brainer.

I certainly understand the time and space constraints but I still think that working at waist level is a lot easier and will generally result in a better end product. I'd also use a quality water based finish like those from General Finishes. At normal room temperature and humidity, you can expect the finish to be dry to the touch within 30 minutes and completely dry within 2 hours.

What I would do is use a series of saw horses in each room that would hold roughly 1/4 of the material needed for that room's ceiling. Finish that first 1/4 in the first room, then move on to the second, and third room. By the time you're done in the third room, the first room's material should be at least dry to the touch. Move that material to the floor, placing it on some scrap lumber, and start on the second 1/4 of material. Depending on how much time you have that evening, you could move onto the third 1/4 (which, in theory, should leave you roughly 1/4 of the room to move around in).

You may find that you'll want more than one coat on each board which will require a light sanding between coats and a wipe with a tack rag.

Thanks, RRM1 & Mike. I appreciate the additional angles to consider. Mike, your proposed way to rotate through the stock is a great idea. I'll talk it over with the DH, and see how that could help us . . .

Finish it on the saw horses before you in stall.

If the planks are more than a few inches wide you WILL see unfinished places when they contract.

the link below is to the "Wood Engineering Handbook" Chapter 3, that details wood movement.

No finish is going to stop the movement.

Wood does not move significantly in length but does move in width. thickness, and shape (see figure 3-3).

T&G construction is one method that allows for movement without creating problems (buckling an splitting).

Finishing every surface (including the tongues) will slow the movement and help make it more uniform during changes.
An unfinished back would allow faster changes in moisture content that can lead to more changes in shape (especially on flat sawn wood, the top inset in figure3-3).

Here is a link that might be useful: Wood Handbook, chpter 3