Welcome to the cbid forums Bill.
I will try and answer some of your questions.
Question 1. Assume a 250 capacity humidor, decently but perhaps not fully seasoned. It seems to be at about 60-65% RH. Will that go up over time, assuming enough humidifier sources inside? (i.e. will it keep seasoning even when in use?)
YES, it will go up, overtime, if your PG mix, is right, for 70%. Many of us, prefer our smokes, in the 60%-65% range, and maintain our humis at that level.
2. What do you think of humidistat tubes and jars, vs. the blocks?
I have not used the jars, but I find the tubes are great for my travel humi. I prefer Climaxx beads, over the various gels, because of the ability to remove excess humidity, and the fact that they come in a 65% variety, that I prefer.
3. If the humidor is pretty well filled with cigars, does that affect the level of RH inside?
Yes. I like to keep my desktop about 3/4 full. The cigars, help keep it more stable, because they hold humidity as well. But much more than 3/4 full I begin to worry about the airflow being constricted. If you are more than 3/4 full, you may want to rotate your cigars once a week.
4. How filled to capacity is a good idea? It would seem to me that there would be less circulation with more cigars inside. Is that a bad thing? 3/4 IMHO And Yes. Re-read answer to 3.
5. Stupid question: opening the humidor drops the RH quickly, at least according to the digital hydrometer. How long should it take to get back to 70% or so? Minutes, hours?
Not a stupid question, but there are way too many variables here. Size of humi, or cooler. Begining humidity, humidity of the room, outside the humidor, how filled the humi or cooler is. and many more variables, too many to list. My desktop recovers fairly quickly, but my coolers take up to an hour to stablize.
Hope this helps.
Dave (A.K.A. Homebrew)
P.S. Welcome Again to the cibid forums.