1. I believe you will get one free month the first time you register it with your credit card. I do not know how the 14-day trail period works, though, and I'm typically wary of it since it's used primarily by RMT as a means to advertise in-game.

2. The only fee you have to pay is the standard $12.95 per month. Not sure what the cost is in Euros. Additional fees (such as the charge for tetra master and additional characters) are optional.

3. You will have to cancel your Content ID at the end of September so that you are not charged in October. You are billed automatically on a monthly basis. You cannot establish ahead of time that you will only be paying for a certain period. If you fail to cancel your Content ID (a relatively easy process) before October 1, you will be charged the full fee for the entire month.

4. Your debit cards should work just like your credit cards. Just keep in mind that if you don't have the funds available in the account before they withdraw them at the first of the month, you'll have a hell of a time trying to get your character back up and running again.

5. This isn't something you addressed, but I think you should be aware, if you're already not, that this summer Final Fantasy XI will be undergoing some serious overhauling. In June the first of three planned level cap increases will be implemented. The second increase is scheduled to be implemented right when you plan to rejoin. This may make getting low level parties difficult, and since you only plan for playing a short period of time, there's a good chance that this will impede your ability to progress in the game more so than usual. I'm by no means trying to convince you not to play, but it is a reality of which you should be aware before rejoining.

Also yeah, I wouldn't count on Square Enix making Final Fantasy XI free any time in the foreseeable future. The most recent announcements regarding the game have cemented that they plan to keep the game going on their servers for a while yet, regardless of FFXIV's impending release. I don't see free, private servers any time soon.