I would think you could add more sand in small increments. I would add a very little each week, or if you have no critters yet, I would get it all done now and wait it out.
As far as LR, if it is cured and it gets to the tank right away, you could be ok. But I would cure it in a separate vessel to be safe. Test and do PWC until amonia is 0. Same as the sand if your tank is empty though.