I think that perhaps the beta blocker is blocking other adrenergic signaling that would normally stimulate your liver and cause it to release extra glucose. Your liver is insulin resistant (from all you've posted that's what I think anyway) so it ignores insulin normally and keeps cranking out sugar in the morning. I too have my BG go UP when I exercise in the morning and DOWN in the afternoon. I think it reflects the liver's extra action in the morning (under the influence of stress hormones and norepinephrine). The Beta blocker is blunting NE's effect on the liver, I'll bet. In fact, I just double-checked this and sure enough this is why your Beta blocker lowered your FBG.
Also, you are hyperinsulinemic and oozing out insulin when you eat, which is why you experience your BG to DROP after eating carbs. I bet you have some type of reactive hypoglycemia (I just discovered it myself, so perhaps though I am seeing it everywhere now.... that said, it is pretty common - far more common than recognized medically at this point). I almost think of prediabetes/reactive hyperglycemia as the same thing at this point. When you eat lower carb, you are not triggering insulin and paradoxically could see a rise from the stimulus of just protein. I doubt it's a big rise, though? The problem with the veg/fish meal not keeping you full is there's no fat! You need fat and protein in the meal, not just carb and protein.
I also understand your pain looking for a cure with a whole-food healthy carb diet. However, your metabolic machinery is clearly damaged at this point. You no longer can tolerate carbs the way you may have been able to when you were a kid. Perhaps you "abused" your metabolic machinery (that's what I did), or perhaps you were destined to have it wear out sooner than the coworker that thrives on a vegan, carbolicious diet 24/7 (yeah, those are all my coworkers).
Ultimately, though, there is a grieving process. If someone told you that you were allergic to something, you'd avoid it right? That's what we have to do with carbs (well, at least minimize them). Now, there are approaches to allergies where people try to "train" themselves to become desensitized. That's what you've tried - you've given it the old college try - and you are not getting more tolerant. Indeed, your BG control is getting worse. Time to take low-carb action! Embrace the beast. Find some foods you love, read some good books to get over that fat phobia.
But do it fast, before you progress further down the road to T2D.
Good luck!