Answered question
If I understand correctly, data upgrades once applied will be received each month and the unused portion gets rolled over. Correct?
Yes, that's right. It's basically like you get "extra data" each month with a 90 days "time limit" to use it.
Other answers
Yes, you're exactly correct.
No, it is not right. Your unused data will be roll over; but the upgrade data only receive after the last upgrades used. For example, if you applied 500M upgrade, but didn't use them last month, you will not receive another upgrade 500M data this month.
@FC you should contact the support, check my image