No. Not everybody is guessing. There's a powerful line of logic that says the odds of mammaries on Mrs. T. Rex are extremely remote. It's not just the lack of soft-tissue evidence.
Your characterization is accurate for ID. That's the problem. With ID it is just a guess.
I also think it's odd that none of the ID proponents will even acknowledge the existence of the evolutionary logic by reproducing it. The score now stands at 0 for 3 for the experts who happily force predictions on evolution that it doesn't make for itself all the time. ("The fossil record should be nothing but transitionals." "Where are the instances of snakes turning into birds?" "Where are the pre-biotic soups forming today?")
OK, it's not a guess. I'll call it informed speculation. If I had to put money on it, I'd say that the mom T Rex was a flat-chested, egg-laying reptile. But there is nothing wrong with thinking outside the box on this matter either and there are enough missing pieces to allow for it.
There is a powerful line of evidence showing that species with mammary glands give life birth - all known mammals except the monotremes. Nevertheless life is far more bountiful, far more different, than your ideology provides for. This is only partially about mammaries in dinosaurs. The main point is that paleontology is circular reasoning, it is a self-fullfilling prophecy. It never finds anything new, because it is predicting from the old. It gives assurances of similarity, when very little is indeed known. Whole species, entire skeletons, beautiful drawings, missing links are created from a tooth, a broken skull, a few ankle bones, or a jaw. Paleontology is not science, it is fairy tales. As to the professionalism of these jokers, just look at my post on the Dinosaur and the Turkey sandwich.