Constraints from LVDS blocks and logic

If the constraints are already there, then you shouldn't be adding constraints(they need to be added in the case where they're not added). Anyway, rather than go back and forth, create a quick schematic/HDL file with the LVDS block coming in, feeding some registers and going out, or something like that, the PLL(if you instantiate one) and the .sdc file that reflects what you have, archive it and attach it. I generally don't look at designs(not enough time), but it should only take a minute or two and faster than posts asking questions.