When Kim said the AV was not painting her fingernails in the bathroom, perhaps Kim was telling the truth.
Assume for a moment that the defense version is actually the truth -- then the defense is only guessing that the AV was painting her nails, because only Kim and the AV were in there, and none of the lacrosse players.
The defense belief that the AV must have been painting her fingernails when the two girls were in the bathroom appears to be based on the fact that when the AV fell down on the back porch, she smeared what appeared to be fingernail polish all over the railing.
Well, maybe it wasn't fingernail polish. Maybe it was lip gloss or some other item of makeup. In the photo at 12:30, she appears to be fumbling with her purse. Perhaps she pulled out a lipstick or a makeup compact and, as she stepped on the loose rags on the second step, lost her balance and grabbed for the rail, smearing lip gloss or other makeup on the railing.
It has always struck me as odd that the AV would be painting her nails in the bathroom anyway, at a time after Kim has already decided the "show is over!" and is in the bathroom putting on her clothes. My only thought was that perhaps the AV was trying to convince Kim to go on with the show, and was painting her fingernails in expectation of winning the argument and continuing the act.
But anyway, why does someone who does her routine in press-on nails need to paint them anyway? Don't they come in different colors? Don't they just put on the ones in the color they want?
So maybe it's not fingernail polish on the back railing. The defense ought to take samples of that and have it tested to figure out exactly what it was the AV smeared on the railing as she fell.