How is the renal osmotic gradient maintained even though the blood osmolarity changes?
The water doesn't get drained.  As blood flows down a vas rectum, in any given section of the arteriole the tonicity is slightly higher outside the arteriole vs inside the arteriole.  This results in a small amount of water flowing out of the arteriole to attempt to equilibrate the osmolarity.  But before this has even finished happening, the blood has flowed down another quarter of a millimeter (or whatever) where the tonicity is even higher outside the arteriole.  In other words, down the entire vas rectum the water is flowing out.  But when the vas rectum comes back up, the situation is reversed, so the same quantity of water ends up diffusing back into the blood vessel.  Thus the osmolarity at any given level of the medulla remains constant.

Correct, the maximum osmolarity of urine is 1,200 mosm/L.  The concentration gradient in the medulla will change very slightly (temporarily), but don't forget that even as the urine in the collecting tubule is being concentrated to that maximum concentration, the whole process of creating the concentration gradient is actively happening in loops of Henle all around the collecting duct.  So even if the initial effect of having a bit more water in the medulla from the collecting tubule would be to slightly lower the extracellular osmolarity, in the grand scheme of things it changes nothing since the rest of the nephron is still actively creating an effective osmolarity gradient.



References

Any physiology textbook... for instance (among others), I have Human Physiology, from Cells to Systems - in the First Canadian Edition it's pp. 553-63.