1. No. Some universities provide institutional access to Zotero that gives free storage to all affiliates. Also, note that you don't need to use storage to use Zotero and you can use WebDAV as an alternative to Zotero storage for personal libraries. https://www.zotero.org/support/sync#file_syncing

2. The prorated amount of your existing subscription gets added to the new one. Depending on how much you have left, that will extend the end date of the new subscription or shorten the end date of the existing one. E.g. if you buy a 2GB subscription but then immediately update to a 6GB one, I believe Zotero will not have you pay immediately, but simply set the end date of your subscription to 4 month (since you've paid for 1/3 of a 6GB subscription). If you use 2GB for half a year and then extend to 6GB, you will pay $60 immediately, but your subscription will be extended by 2 month (since the remaining 1/2 -- $10 -- of your 2GB subscription is 1/6 of an annual 6GB one).

3. Zotero sync doesn't protect you against human error, so no. See https://www.zotero.org/support/zotero_data#backing_up_your_zotero_data