The only definitive information I could find specific to your question was in the book Malts and Malting:
'[Malt] must be stored cool and dry in sealed stores [...] to arrest the decline in enzyme levels'
One brief and somewhat vague sentence in 750 pages may give you an idea of how little professional maltsters and brewers seem to concern themselves with loss of enzymes/diastatic power throughout the course of proper storage.
Here is another study published in the Journal of the Institute of Brewing which investigates changes in malts over a period of 12 months at different temperatures and humidities. In it, the conclusion is drawn that:
'The sugar content in the sweet wort was constant throughout malt storage, showing that the major components in the malt and the starch-degrading enzymatic activity were intact.'
However, from a theoretical standpoint enzymatic activity (and therefore DP) absolutely does decline with time. Thanks to entropy, we can be relatively assured that enzymes will have a tendency towards denaturing (a less ordered state) and not spontaneously "re"nature themselves, resulting in a net decline in activity. Luckily, when stored cool and dry this will be an incredibly gradual process, on the order of years, maybe even decades. But we also know we have to deal with two factors which accelerate enzymatic loss: elevated temperature and elevated moisture.
Let's think about this a bit more, as it applies to your malt-in-question. The study I linked above looks at the effects of malt storage at either 10 or 20 degrees C (50 or 68 degrees F) at a different relative humidities which equate to roughly 8% and 12% malt moisture, showing no significant loss of enzymes. You say maybe your grain hit 80 degrees F. Could this account for a significant loss of DP? That depends on how long it sat there, but it certainly will be worse off than if it hadn't gotten that warm.
You also say it was stored air-tight but, over the course of a year and a half and with (presumably) multiple openings, your malt could easily have achieved (and almost certainly did) a much higher moisture content than it started with. For example, given my local climate, assuming a year-round average relative humidity of 65% (for simplicity) I can safely guess that malt stored for a significant length of time here might eventually reach ~12-14% moisture, as it comes into equilibrium with the environment. For reference, you'd expect a fresh and properly stored base malt to have a moisture content of ~4-6%.
It's well established that enzyme destruction is much more significant at higher moisture contents, given a certain temperature, so I think it's safe to say that with the conditions you describe there was certainly the possibility for significant loss of diastatic power. Also, as you point out, beta amylase is a more sensitive enzyme and will be prone to a greater degree of destruction in a given set of conditions. This seems to be born out in the specific issues you are having with the malt. Regarding loss of enzymes to direct-fire heat, it stands to reason that a malt which is already enzymatically 'compromised' will suffer a more drastic loss of DP than a more sound, fresh malt.
Basically, it's easy to say that, theoretically, yes DP should reduce with time (albeit very slowly), but is incredibly difficult to say to what degree it is lost in any given situation, for any given malt, even if you know as much as you can about the storage conditions and the state of the grain. I think the only fool-proof way to answer that question would be to perform an actual test of the diastatic power on the grain in question.