Lambic Temperature Fluctuation

I wouldn't worry. Heat expedites aging, mainly oxidation, but slow and steady movements that don't get that hot aren't an issue. On top of that, there aren't any hops to speak of in lambic so you don't need to worry about losing them. And, to a certain extent, it's oxidation that you're actually looking for - not the "cardboard" kind, but things that you don't really notice as oxidation. Increased maltiness is one, but you probably won't be able to detect that in lambic, what you'll find is a softer roundness.

As for cellaring, I've never aged lambic, I always just drank it right away but I've cellared many beers in similar conditions to yours without ill effect - and to be honest, as you've pointed out, this is pretty much the norm. If you've got a cellar that's deep enough or long enough that you've got a constant 55F you've got it made, but most people have some fluctuation.

I'll agree with you that cellaring and storing in a refrigerator gives the same result, at least as far as 99% of earth's population goes concerning tasting, the only issue is moisture loss on corks in a fridge. Any bumps in temperature in the cellar probably only speed up the process by a tiny bit.

On the other hand, I should note that cellaring was originally done simply for storage purposes, and not to improve the wine (or beer). That came later on, most likely based on noticeable improvements, but was also most likely based on an inferior product to what we have today. Good luck! Have fun with it.