Perhaps you won’t know the answer to this, but was the place re-plastered before the carpet was laid?
My grandmother, a fix-it-up-yourself landlord, once took the carpet up in a house she had purchased because she felt many lumps beneath the carpet and feared for the integrity of the flooring. It turned out to be hardened globs of plaster which had apparently plopped onto the floor when the previous owners were filling holes in the walls, and just left there. That explanation might also account for your “crunchiness”, and for feeling cracks.
I’d say to take a long pin, like a hat pin, and try sticking it down through the carpet over one of the cracks. If you don’t hit anything, you’ll know there’s a problem.
Having bugs doesn’t necessarily mean the flooring is cracked. Bugs are remarkably determined to enter human dwellings and very adept at it. Some areas are just more bug-prone than others. I just moved into a new house. At the old one, we had an infestation of earwigs (those horrible little bugs with what looks like pincers on their butts), and the neighbors complained they struggled with them, too. In the new house, I haven’t seen a single one. My grandmother and her neighbors have problems with ladybugs, and despite strenuous and elaborate efforts, have been unable to keep them completely out.