Merry Christmas. I'm not a staff at PGB, so take my response with a grain of salt. Adobe folks, please correct me if anything I say is wrong.
1. As far as I know.
2. I've never tried, and I'm not staff, so I don't know how the limit is calculated.
3. IIRC, you have to pay for the full Adobe CC plan ($49.99) to get 1GB.
4. I don't know what you mean here -- do you mean the PhoneGap Desktop App (not related to PhoneGap Build), or do you mean building for Windows using PhoneGap Build? If the latter, the size limits should be the same. If the former, the Desktop App doesn't have any enforced limits, and is free to use.
5. The amount of memory required by an app is highly variable and dependent upon how you code the app and how many assets you have loaded simultaneously. If a memory leak is present, it'd be entirely possible that your app could consume significantly more memory than the app size. There's no good way to answer this question without specifics.
6. There's no good way to answer this either, since it depends upon the compression ratio of the app bundle. The OS will decompress the app bundle, so at minimum the space required will be the expanded app size, but there may also be file system differences that would result in size variations as well. Furthermore, if your app stores content locally, you'll end up consuming additional space.
I would seriously question whether or not you need to create a 1GB app bundle. You /can/ download content after installation using various means (File Transfer plugin, code pushes, etc.). I suspect you'll have nothing but trouble at 1GB even locally (you have to upload that bundle to an app store too...).
for creative cloud subscribers the max upload size is 1GB
max app size is calculated on the compressed size of the app. this is after it has been unzipped/cloned and any hidden or source control files are removed (./.git etc.)
i am not sure if it possible as a single product subscription. it is either 9.99$ a month for single product subscription (100mb limit), or as part a full creative cloud subscription (50$ a month).