Lets see if I can help.......
#1: No, you will not be able to cut a sharp inside corner. There will always be a radius due to the round nature of the plasma stream. Example... Can you drill a square hole with a round drill bit?
You may be able to cut a relief into that corner by putting a small diameter circle overlapping the corner.
2: I ran into the same issue on a couple projects. I essentially "tricked" sheetcam by entering the kerf width smaller that it really is.
Otherwise, dont draw it as a slot, draw it as a line. Move the lines to another layer and cut them with no offset.