1) Is the any advantage in using a grain magnifier to check sharpness on the baseboard over say a 10x traditional magnifying glass?
The grain focuser is a lot easier but if you can do it with a magnifying glass, that's fine.
2)I can't afford a precision spirit level to align my enlarger, I read a suggestion on another post that said to sand down a negative and use a magnifier to check for uniform focus while making corresponding adjustments. Has anyone tried this? I don't mind if its a little time consuming as long as it is effective.
I've never done it any other way. Scratch the film with a needle, making a grid pattern, for maximum ease of focus. Wipe the scratced neg with your thumb to remove loose shreds of gelatine.
3)The longer you expose the print the grainier it gets
No, or at least, not visibly. You'll never see the grain of the printing paper. Overexposed negs get grainier, but that's another story.