2 things. When I tell Pretty-Boy Lloyd to go in the grave and then drop the mine on him it won't let me go down in the grave. Secondly, if I just tell him to go down without dropping the mine he will not be there and my game freezes when I go down.

I had that glitch before along time ago, I think I fixed it by either reloading or had to find a previous save or something. Worst case scenario the save game file is bad and that glitch isn't going to go away unless you start over or have another save with your character on another slot...

I do remember this happening to me also.

I've tried reloading

Yeah, its buggy.

I don't remember if this one is solved by Killap or not in his unofficial patch.

Your options are to use one of the options (have him dig and don't drop the mine) or just simply pay the debt out of your own pocket.

(And people winge about FO3/NV's bugs...)