Regarding the cardboard, I do agree it can be problematic for some rabbits if the eat it. Some rabbits can get away with eating tonnes, but I have seen cases where it’s slowed the gut, including in one of my own rabbits.
When she had more cardboard, did you ever see her eating it?
If not, perhaps it’s something she can have again but only when you’re home and she’s in your line of vision.

You mentioned she’s middle aged. Has she had a thorough dental exam? To truly get a look at the molars, it would probably require sedation. So often, de real issues can be connected to GI issues. Also, other unidentified sources of discomfort in the body.

You sound pretty rabbit savvy, so you’re likely already aware of that!