You really do seem to have eliminated all the obvious stuff so I'm rather grasping at straws here but, I'm wondering if there might be an internal fault with the caliper (I can't think what though!) Is there any way you can lay your hands on another one and try it in it's place? If you get the same result then you can pretty much be sure your existing caliper is not the problem. - Maybe a forum member could lend you one temporarily?
When bleeding does it bleed well or is the flow poor? (compared with the other side). I'm scratching my head here because you seem to have tried all the things I would have but I'm wondering, if you can totally eliminate the caliper as a problem, and bearing in mind you've replaced the flex hose - which I have seen cause this sort of problem when they internally start to collapse - Whether you do have an ABS unit problem. I can't see it being the sensor because it sends a signal to the ABS ECU which the ECU then compares with the other 3 outputs to compare rotational speeds between the wheels. As you say, if there was a problem you'd expect it to light a warning light and it shouldn't have any influence at low speeds anyway. HOWEVER. I think there could still be an internal problem with the ABS unit? Do you push the caliper pistons back without opening a bleed nipple? This might, especially on an older vehicle where there is a high likelihood of contaminants in the fluid, cause internal ABS unit problems with valve units etc? I have to say that in newer vehicles I have no problem with pushing the pistons back in the time honoured fashion, but once over about 5 years I like to always open a bleed nipple and allow the fluid to bleed out rather than being forced back up the pipe.
As I said above, I'm grasping at straws here, but I think if you can definitely eliminate that caliper by substitution then that will take one of the big question marks out of the equation?
Sorry I can't be more helpful. Good luck.