My installation of multisites was interrupted yesterday during the Joomla install process. I have solved the issue (mysql password) but I have some concerns about moving forward since I didn't create a backup copy of the master site. The master was installed under localhost/joomla15 and now I can’t back into it without continuing the Joomla install process.

How do I resume the setup process as Multisites intended? Am I correct in the following:

Running the Joomla installation for localhost/joomla15 will not overwrite the master site if I change the table prefix from jos_ to another prefix AND create a unique database for the site. Or do I just need to do one or the other (change prefix for shared database or create unique database name for separate databases).

I would feel a lot better if I had installed JoomlaPack and created a backup first, but need to know how I should proceed without losing the original site. Should I run through the Joomla installation for localhost/joomla15 but use the database name and table prefix that I entered in the multisites program before getting interrupted?

Normally, the JMS configuration should not be impacted as it save the configuration into files on the disk.
If you want to perform a backup of JMS data, you can backup the /multisites directory.