There's a few things worth considering when it comes to breeding corn...
You're right that it can adapt quite quickly to an environment, but only if it's starting as a diverse population. A extremely uniform variety will show very little adaptation.
Corn hates being inbred. If you keep growing out seeds of the same small population, after a few generations you'll see a significant reduction in plant health and yield. This is very similar to the effects on humans when a small population inbreeds for some time.
To prevent this, ideally you want to grow a relatively large stand of corn. You will here a minimum of 100, 200, or even 400 depending who you talk to. Personally I think 400 is sufficient.
If you are unable to do this, you can mix seeds you harvested in with ones of the same variety from your seed source, or another seed company, and this should prevent the bottlenecking from occuring.
Corn can pollinate other plants from very far away, much farther than is practical for most people to isolate varieties. The best way to grow two varieties is to plant an early season and a late season corn, this way they will tassle at different times. For people with long growing seasons, they could also stagger their plantings (but I doubt that applies to you).
As for the old seeds, you could try soaking them in a dilute hydrogen peroxide solution. I believe 1% is the normal strength, but you should doublecheck before trying. That said, it would be very difficult to extract those genetics unless you were able to germinate 100 seeds or more, or if you chose to cross it with another variety
