"Inside" to me means where the glue was....any DNA in the garbage can, on the floor, etc., would adhere to it. I'm not assuming the glue was still wet, but nevertheless, that part of the nail evidently had DNA on it....according to the findings. IMO....doesn't mean that anyone is guilty of a rape.
Well, UNDER is almost universally used.
I suspect somehow.. even maybe as you suggest it's on the underside but not at the end of the nail (on the rounded end)
Under artificial nails, I found this:
Coat the inside of the artificial nails and your own nail.
Inside may or May not be consistent with Under I guess.