This is something where conditions will affect your outcome greatly.
"Some hot and humid weather and some below freezing temps..."
Who knows what that means?
Sprouting jars are cheap. You can buy a kit for ten or twenty bucks. We use them to grow sprouts for the kitchen. And, any questionable seed for gardening/farming gets tested in a sprouting jar in the spring.
I was pleasantly surprised last spring, when it was too late to order dent corn for our garden; my wife shelled the remaining dent corn that we had hanging, on the cob,in the house. That corn had been hanging for 2-1/2 years at that point. We got about 1/2 of a five gallon pail of kernels; and when I tested them in the sprout jars, they passed. That much seed would have cost well over 100 bucks. And, we just harvested a nice batch of dent corn for homemade technicolor johnnycake last month.
So far, I've tested a few types of seeds that were leftover from previous years, such as: beets, peas, dent corn, lettuce, etc. and all of them have passed with up to three years' of storage.
Even so, I wouldn't plant old seed without testing first.
That's my reasonable answer.
My old school farm boy answer is: One particularly wet summer my brother baled the "big-meadow" at the home farm in late June; but couldn't get to second cut until September. In the meantime, July and August were wet. By the time he got to the meadow, it was a carpet of red clover. My father (born in 1928), remarked at the time that "the old timers" (born who knows how long ago) told him that clover seed was "hard" and could "lay dormant in the ground for years" before a long, wet summer would get it to germinate. So, I guess, according to the old timers, clover seed is pretty hardy. Myself, in my personal experience, I think that clover seed is hardy; and actually benefits from a little scarification/inoculation... I seed clover by outfeeding the herd with bales of clover hay during late fall, early winter, on ground that I want seeded to clover. The cows digest the seeds, scarify them, inoculate them and deposit them on the ground... they also trample some into the ground. If I had clover seed... I would probably just broadcast it in the areas where we keep the cows in the fall, let them trample it in during the fall... and hope for the best.
I also think that the longevity of a clover stand depends heavily on management. If it's a mixed stand, it seems that you need to get the grass off in June or early July... to give clover its time to dominate during the hot weather of July and August, where grass suffers. Otherwise, if you let the stand get old a few years in a row, the dead grass will smother it out.
