Yes I would believe so.
If you have the full game downloaded into your computer, no. All you have to do is make the trial account and when it says to download it dont, and just get on your trial account.
Yes.
No, once a trial account has been upgraded to a full account, you can not go back to using it as a trial account. If you want to do another trial account, then you need to sign up for a whole new one.
For most private servers you have to make your own account on the server's website.
Go to the official site linked below. Create an account. Add WoW or a WoW trial to your list of games. Download client. Log on with your account, and create your character.
A trial account itself is not the issue, it's the installed program (client). Once you have that installed, and you have made an account with a private server, it will work regardless of you also having a trial account at Battle.net.
Technically, yes, but it would be better to download the whole thing if you have a regular wow account. The trial client is a streaming client, meaning that not everything is on your computer. It will be a lot slower, laggy, and things will take even longer than they normally do to load.
Yes, you can use any addons that a full account can use. There is no limit.
If you tried to upgrade your trial account but accidentally created a new WoW game in your battle.net account, the best thing to do is contact customer service. Explain them that you would like the characters from your trial account moved to your new account instead. If you had wanted to use the RAF feature instead, instead tell them that you would like to make use of the RAF feature and that you would like to know if it is possible for your trial account to be upgraded with the information in your second account, to keep the possibilities of the RAF feature. In either case, it is likely you will be asked for identification and the authentication key of your new WoW account, so keep them close when contacting them.
There are trial accounts that allow you to play for 10 days to try WoW out but that's it.
Yes, you should be able to play the test realm/client using a trial account.
It can take several days after you finish processing the upgrade. Give it a few days.