I am NOT confused abut the size at all, so don't know where you got that impression. It is in the interview transcripts that police told Patsy that NO other panties size 12 were found. As far as I know, the panties were labeled size 12 (Child's). Those kinds of panty sets are usually sold in a variety of children's sizes 2-3, 4-6, 6-8, 8-10, 12-14. Girls' sizes usually stop at size 12-14, then smaller women's sizes begin. Women's panty sizes run differently. There are sometimes S, M, and L. But most women's panties are sized beginning with adult size 5, then 6,7,8,9,10,11,12. A woman who wears a size 12 panty would be a VERY large plus-size woman. The panty size 12 (adult) does not coincide with a woman's dress or slacks size 12.
While it was certainly possible for the panties to have been tested for detergent residue (actually, I had discussed this here a long time ago,before you came here) I doubt they were. I had not seen anything indicating that. As far as I know, they were taken right out of the package.
I doubt all JB's panties were dirty, LHP had been there just 2 days earlier, and one of her jobs was laundry. The police said in their interviews that JB's panties were all stained with fecal staining. Doesn't mean they hadn't been laundered. But if they were not pre-treated or washed with bleach, those stains wound not come out in the wash. Many girl's panties are colored prints, and if they were bleached, the prints and colors would fade, maybe that's why they weren't.
I think she was wearing the big panties because they were right there in the basement when they were needed, and therefore it was unnecessary to go upstairs to find a clean pair of her own. This would be problematic for an intruder, who would not be familiar with where she kept her underwear, as it was in an unusual place, and would not know there was a new gift set of panties right there in the basement.
I think it is as ludicrous to believe someone took the panties out of the house (they had been wrapped, remember, and no stranger would know that) and brought them back as it is for the R's to say that the flashlight that looked like theirs but wasn't because it was "dirty" (because it was taken out and brought back). Just how many times would this SFF have come and gone from the house unnoticed?
As far as why the longjohns were not bloodied, my theory is that they were removed for whatever caused the vaginal bleeding. There was blood (though a small amount) found inside her vaginal area (on the forchette) and in other places as well as traces of her own blood found having been wiped from her thighs and pubic area. SOMETHING made her bleed there, as 6-year old girls do not menstruate, and the coroner stated all her reproductive organs were pre-pubescent. THAT is what is a puzzle. Was it a too-thorough cleaning after soiling? Or was it a sexual assault. That is one of the problems I have personally with this case. If it was a sexual assault, I really can't see Patsy as the abuser. I can see the brother(s). I can see JR, possibly. And yes, here are a limited number of others I can consider. But if it was sexual abuse and not vigorous douching (which is only Patsy, IMO), it also raises the questions of the paintbrush. Was it inserted AS the assault, or was it inserted as a coverup for either the douching or digital penetration (as Mayer suggested to Det. Arndt he thought was the cause of her injuries).