[UNRE4L]

I can answer the first two questions, but i'm not sure about the third.

So long as you use the subscription manager to edit how long it lasts for it will add/remove the subscription when necessary. If you are doing a promotion though why not just change the "Subscription Length" field in the options for each promotional subscription? It means that if you are doing buy one month and get one month free any purchases that take place during your promotion will be given two months instead of one. When the promotion ends you change the length back to normal any purchases and it will go back to only one month per purchase. Subscriptions bought when it was in the offer phase will still have their two months and will expire when needs be.