I'm just a plain ole IBS-A gal here but I've been dealing with this dopey "syndrome" for nearly 30 years. D has never really been my problem, at least not to the degree that you've described. When I have it, usually 4 or 5 times in a day is the max (and I am always grateful for D!).
Regarding pain, I and many others do experience it with great regularity (no pun intended!). It's a daily occurrence for me, regardless of the quality or quantity of BM's. While there may be some "break" from the pain after a particularly "wholesome" elimination, usually the pain returns within an hour or so. I suffer with trapped gas that causes tremendous pain. My gut is a gas-making factory - in production 24/7! It is not uncommon for folks with severe IBS to experience daily pain.
Bentyl, IMO, is the single most useless medication I have ever taken (and that includes all medications across the board!). I have no idea why doc's still prescribe it. I have never had any success with the anti-spasm drugs and in fact, find they worsen C in my case.