We started using K2 in January of this year and have been doing very well, when the new version came out, I mirrored the site and installed it as a test, did my modifications and found it to work fine, so I went ahead and did the installation on the live server.
That killed everything, the site spat errors all over the place and in the admin section we couldn't edit any articles or do anything related to K2. Apparently the upgrade didn't go well for some odd reason. I started troubleshooting and replacing files and code all over the place and now we have a semi-working website where 90% of the front-end is functional and 75% of the back-end is functional.

We have around 9,000 articles, some with video, most with images, which I can't lose and now need to somehow fix this. The "test" website can't be brought live because there's five days worth of content on it (this is the time it took us to fix the messed up upgrade) thats missing.

My plan now is to restart from zero and do a clean Joomla + K2 reinstall on a third server and then import all the articles from the messed up install into it. Yet I can't find any simple way to do this.

Can I just grab all the jos_k2 database items and merge them into the new site and it'll work or is there some massaging to be done to the db files?

Yes, export your Joomla database.
This will contain all your joomla data such as modules used, menus and k2 too.
Consider getting Akeeba backup to help with that task. I am still old school and like to do things manually.

Get your new Joomla and K2 site up and running.
Import your data.

You should have no issues, fingers crossed as there will be no info int the db that would cause conflicts.
It might also be worth taking a look at the Joomla documents as that will advise on backups too.

After that, you would then probably need someone to take a look, as doing would be quicker than writing and waiting for a response.

Simon

K2 Support