I have seen a lot of fish keepers out there that talks about having to keep corys in groups of 4 or more. After doing a couple days of research, this seems to be a ploy by the fish industry to get you to buy more than you need, nothing more.
There is some truth to it in the fact that if you are going to have more than 2, you need 4-6 of them to allow them to pair off and to estbalish their social order in the group. If you want to breed then, groups of 6 or more are better and equal numbers male:female is supposed to work just fine.
You can have 1 or 2 of them in a tank and they will be fine, probably extremely happy if you have places for them to hide in. I have seen them raised alone and they were some of the most colorful and healthy corys I have ever seen. A single male and single female will actually interact with each other like a couple sharing and taking turns at different things. I know becaus eI have done this and was finally able to identify the species for certain because of the coloration finally matching the pictures I was finding.
There is no reason to believe that a single cory will be so depressed and stressed that it will not survive. They are found in groups and also found in single throughout their natural habitat as I have found on many sites.
If you plan to have a wide open bare tank with just gravel I would say that that is about the only time that you really need to have a shoal of them because they will feel like every other fish is a predator and shoal together in a defence posture which is probably more stressful than keeping them as a single cory in a tank with some plants or some sort of cave to hide in.
As for life spans, I know of at least 6 different fish keepers who have kept them in singles for many years and they outlive the ones they have in shoals. They don't know why but the fish does not care if it is a single fish or a group of 20 in a 125 gallon tank. As long as they have places to hide they are happy regardless of their numbers.