This should help you:
Everything below is based on my experience using Drive.
1 - As long as you don't hit the limit, Google Drive will be free. It doesn't matter what files are there, as long as they are less than 15GB. Keep in mind, that Gmail messages and attachments count towards that quota. You can check your usage by accessing Google Drive online.
2 - I don't think it's a "trial period". If it is, 5 years is a hell of a trial.
3 - Theoretically, only you, unless you share them with others. I say theoretically, because the files are on a computer that isn't yours and connected to the Internet.
4 - Logging out of the Google account on the drive agent, not the web browser, will only stop drive from syncing. It will not make the local files in the drive folder inaccessible. That means yes, if someone else uses your computer, they can access your files that are locally in drive. It just won't sync to the cloud.
Signing out of google in your web browser has no effect on the drive agent running on your computer.
5 - Yes
The Google Drive Folder is a folder on your local PC which syncs to Google Drive on the web. Anything you place in there is uploaded to the cloud. If you delete files from that folder, they also get deleted from the cloud.
Now let's say you have "document.docx" in your Google Drive folder and a copy on your desktop. Those are two separate, independent copies of the same file. Changes made to one are not reflected in the other. If you delete the one in your Google Drive folder, the one on your desktop will not be deleted. The reverse is also true.
As I said before, logging out of Google via the web browser has no effect on Drive. If you disconnect your Google account from the Drive app on your computer, then the Google Drive folder stops syncing with the cloud. The files on your PC are not deleted or otherwise made inaccessible.
Hope this helps.