This must be so stressful!
Posturing like loafing and pressing the belly against the floor are both likely indicators of uncomfortable gas. That may explain the "rumble" sounds you describe.
When I think of gas, I consider what may be causing it. You've outlined a diet with what I would consider unusual items for a 12wo rabbit, including all the veggies and sweet items, like carrots and dried cranberries. Typically, rabbits under 6 months old should not be given anything aside from hay and pellets because their stomach hasn't developed enough to properly break down vegetables. When the stomach isn't "working right", gas is produced, and this can be painful for a rabbit.
Mushy poops and runny poops can be a sign of gas as well as low fiber. If your rabbit is eating a lot of veggies and sweet treats, he may not be eating as much hay as he should (which should be around 75% of his diet at this point; people say different %'s, but the idea is, it's very high).
Can you explain what his daily diet has been, including quantities of food?
... Also, diet aside, if it's getting warmer where you live, your rabbit may be shedding. What has his grooming routine been? It could be he has ingested a lot of excess fur when cleaning himself and it has begun to clog him up. Fur can also pass through and be covered in poop, making poops appear like a string of pearls of a longish log. If you break a part a poop and see a thick bundle of fur, then you know this is the case. If it's just a soft, big poop, then concentrate more on what is outlined above.