Please note: I don't believe this question is a dupe. There is this question, which has some excellent answers, but they are suggestions that don't apply to my situation because I have ruined my cast iron oven differently than the user who asked that question.
So last month I got a massive 16" cast iron dutch oven from Cabelas. I seasoned it by cooking it in a campfire for a few hours (w/ lid on), letting it cool, and then rubbing a little bit of olive oil into it. Last week I took it for a test drive and cooked some peach cobbler on it (again over campfire), and it was perfect. I cleaned it by boiling water in it for an hour, scraping off the crust with a wooden spoon, and then reapplying the olive oil to it.
Last night I decided to cook a pork shoulder roast inside of it. The instructions said to let it cook at 325°F for 3 hours. Since I'm still so new to campfire dutch oven cooking, I got a roaring fire going, and unloaded like 30 charcoal brickets into it and let them heat up. Definitely was blazing too hot, but I didn't know any better. (Subsequently since last night I discovered that there are actually formulas that correlate specific numbers of brickets and their locations with specific temperatures. In hindsight I think my oven was soewhere around 450°F.)
I put the oven on the fire (with the pork inside of it, and about 1/4" of apple cider vinegar resting underneath it). 90 minutes later I went to check on it and the pork was totally charred black and overcooked. So it was pizza for dinner! But, to my horror, the apple cider vinegar-herb-seasoning liquid had now turned into black carbonic rock in the bottom of the oven:
Under the lid was a buildup of a super thick tarry sludge that has the consistency of superglue.
I tried washing both the oven and lid with salt water (course grained). Very little came off. I tried boiling the oven over the same campfire for a few hours, and again, practically nothing came off. I went to the Google gods, and the best idea I was able to find was to soak both the lid and the oven in a 50/50 mixture of water + vinegar overnight. So I did that.
The oven remains unchanged, even after soaking in a vinegar solution for nearly 10 hours:
As for the lid, all of the surficial tar/sludge came off, but it is still super sticky to the touch, unlike it was before. And now there is almost a 'metal vapor' that its giving off.
What the heck happened here? Did I just cook at too high a temperature? Are you not supposed to cook apple cider vinegar in a dutch oven over the campfire? Much, much more importantly, how can I fix this and restore the oven to its full glory again?