The point of Green Glue is to create a flexible layer of never fully firm glue between layers of drywall, right? To absorb sound?
In which case, I think you'll need to first level the ceiling, then apply green glue, then apply your final layer.
Otherwise, if you green glue over top of the texture, you'll have high points in the texture that touch the new layer of drywall. That will transfer sound quite effectively - which you don't want.
Condensation is definitely a concern in colder weather. If you create a thermal break that isn't properly sealed against air leakage, warm air from the house will seep behind the walls, bringing lots and lots of moisture with it. Meanwhile the bricks, which are now insulated, are going to be very very cold, and when warm air gets cold it lets go of its moisture - and thus we get condensation.
That condensation would then get trapped inside the walls - and long before you have a problem with brick deterioration you're going to have mold issues that lead to health issues.
The best thing to do to prevent this is to insulate with a substance that also serves as a vapor barrier, which will prevent any air (and thus any moisture) from leaking into the closed cavity. The best product for this IMO is spray foam insulation, because it fills in all the nooks and crannies and IS the vapor barrier.
Alternatively, you could use fiberglass and cover it over with plastic to form a vapor barrier, and this can work IF you properly seal it all the way around the edges, seams and any holes that occur.
Either approach, however, is going to require you to sacrifice living space, as for a proper thermal break you're going to probably want at least 4" of insulation.
Now - with regard to insulating your spare rooms. Don't insulate your interior walls. Put proper insulation in the external walls of those rooms, so that they don't lose heat to the outside. Yes you will be heating them, but if they're properly insulated they're not going to cost much to heat and there are numerous other concerns with turning an inside room into a "cold" room - it's a finished space, after all, and you don't want to risk damaging that by letting it be freezing cold in there.
Best Answer
It would depend on how much effort and cost you are willing to invest. Also how large an area is another factor. One way (depending on your DIY skills) is to cover the texture with drywall. Drywall would need to be taped and sanded and primed and painted. I've removed popcorn texture by spraying it with water from a pump sprayer, letting soak-in, and scraping the texture into a plastic bag. I found a scraper made specifically for this work at Lowes. Scraping with water involves plastic drop clothes and afterwards skim coating dents with spackle. I would think both suggestions take about the same time. Drywall might be a bit more costly and labor intensive. Scraping can be accomplished solo with minimum dust produced. But when removing or working with suspicious (or unknown) material that can be inhaled (like 40 year old texture) take precautions by using protective equipment.