The only way to prevent botulism in canned foods is to either make it to acidic for the spores to survive, or to pressure can it using the right equipment at a temperature of 121C for at least 3 minutes. If this hasn't been done then it could be full of botulism, in which case simply cooking it above 80C for say 30 minutes just to be sure (that's double the 15 minutes I've seen recommended on some sites, I'd take no chances) - as you say will destroy the toxin. If you eat it right away afterwards then you'll in all likelihood be fine, it takes a couple of hours at least for enough toxin to build up to be harmful, even in ideal conditions for it to develop.
However, if there was a chance that there was botulism in home-canned food given to me I wouldn't take the risk. Even though I know scientifically that I would be safe by following the above advice I wouldn't be able to enjoy the food, every bite I'd have nagging worries that I'm poisoning myself. If I was starving I would in a second of course, but I can walk to the corner store and buy all the food I want so why take the risks?
I wouldn't hesitate to try home canned food made by someone who knows their stuff, but if there's any doubt of the provenance of home canned food then the best advice would be to throw it away.
Edit: See my question here. I plan to update this answer after I get an answer to that question.
I'm a beginner when it comes to this (I've fermented a few quarts of pickles with Fermilids), but from everything I've read, you'll probably survive if you eat it. However, if you add too much salt, that can kill the bacteria needed to do lacto-fermentation, and you may end up salt-curing your vegetables instead of fermenting them.
From what I understand, a 6% salt brine solution is about 3.34 tablespoons per quart of pickles. I've read that your brine should only ever be between 1 to 3 tablespoons of salt per quart of pickles (if you're not using a starter, and you can perhaps use less salt if you use a starter), and that you should use more salt the warmer it is (but not more than 3 tablespoons).
I believe the delay is partially, if not entirely, due to the salt. I imagine that the activity is from beneficial bacteria that needed a longer time to get started, due to the salt. I don't know of any bad stuff that would survive those conditions, if you're not getting mold. I can't give you a definitive answer here, but from everything I've read, that's my opinion.
I used 2.5 to 3 tablespoons of pickling salt on a quart of rhubarb, and I didn't taste much lactic acid in it after three weeks. However, with about 2 tablespoons of pickling salt, and a probiotic (a chewable L. acidophilus and B. lactis tablet) my rhubarb tasted pickled after 6 days. Temperatures for both sets of pickles were between 69 to 73° F., I believe.
From those results and the results with my radishes, I personally would be leery about using more than 2 tablespoons of pickling salt for some vegetables. I'm not sure about cabbage, onions, oranges, etc.
Remember, though: I'm new to this. So, you probably know things I don't.
You'd think onions would ferment quickly, because they're known for their content of FOS (a pre-biotic and sweetener). However, from my experience, I know oranges can take a really long time to compost in a garbage can with a lid on it (they're still whole and visibly oranges after a couple years). So, maybe the orange juice contributed. I'm not sure how LABs respond to citric acid.
Best Answer
Unless you know that the alcohol was added by the cook at the time of canning, I’d treat it as a sign that “something” happened. And in canning, “something” is always considered bad, because it means the food was in uncontrolled conditions.
I’d stick to the golden food safety rule of “when in doubt, throw in out”, just to be safe.