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Hi Tony,
The short answer to your first question is: they don't stabilize the bubble for temperature. The various types of bubbles are adjustable at the time of use and, usually, removed, or the bubble system is relaxed when not in use.

Many bubble systems are at nearly atmospheric pressure and have a side chamber for the "air". There is a method of making the air chamber larger or smaller and specific instructions tell you how to position the sextant to put air into the chamber for a smaller bubble or to put air into the sight path for a larger bubble.

Some bubble assemblies use a specific fluid, I think installed under a partial vacuum, absolutely no "air" in the system, and a strong metal diaphragm connected to a thumbscrew to reduce the fluid pressure until a small amount of fluid in the sight path "boils" into vapor and creates a bubble. These are much harder for mere mortals to work on.

There are many posts available if you search for "refill bubble".
Regards, Noell