how to reinstall K2 R6 after uninstall & reset button

I was having trouble getting a clean installation of the header graphic using the K2 theme (the latest version, which is listed as stable, R6). I was getting some strange interference, so I thought I might use that clever uninstall and reset button/procedure. I speculated incorrectly that I could easily reinstall K2 R6, clean and new, and get it to work. Now the site is completely down to customers and I cannot get the K2 theme to show up anywhere in admin view (dashboard), Design link (No longer called Presentation link as most notes suggest). I have uploaded all relevant files again to their proper locations, but there must be some code changes necessary after the uninstall & reset of K2. I cannot activate K2, because I can no longer find it. I have some idea of where I might make changes, like wp-admin\themes.php or wp-admin\theme-editor.php, but I am not sure and replacing those files with older working backups did not solve it. If anyone could offer suggestions to point me in the right direction, I would be most appreciative. The problems for me seem to be compounding. I leave some background below, but if I can solve this, I can post the other problems separately.

background: I inherited this wordpress website: http://www.sancarlostennis.com I had it chugging along just fine, and one day the site simply broke down, throwing a few fatal errors — to my knowledge with no intervention or code changes at all for weeks — suggesting some inherent wordpress instability. To make a long story short, reactivating plugins helped resolve many of the problems, but left the site unable to accept new posts and without a visible Event calendar, and worse yet had the unfortunate side-effect of suggesting to me that reactivating and upgrading was a reasonable approach to fixing the site. Since reactivating the plugins I made the mistake of upgrading wordpress and the K2 theme instead of getting on the forum at that time and addressing my two remaining problems. I usually try to read everything I can before posting. I don’t see anything addressing this specific problem. If I can get out this particular jam, at least that will be a step in the right direction for a change.

I have spent a great deal of time on this, and am poring through the PHP code to determine where I might make a few changes, but any insight would help me greatly