The beaver hair (if it was beaver) could have come from a few things. Patsy was said to have fur-trimmed boots (she "couldn't find them" to give to LE) as well as several fur coats of various pelts. If could have also come from the paintbrush. I never saw that the brush end was tested to see what kind of fur it was. Artists brushes are usually natural fur which is best for watercolors and oils rather than synthetic materials.
The brown paper bag fibers likely came from the brown paper bags that were places over her hands (sometimes they are put on the feet too, standard procedure) by the coroner's forensics team when they arrived at the house. The bags on her hands can be seen in a crime photo, showing her lying on the living room rug. You can't see her whole body- her face is turned away, but you can see part of her hair and cheek as well as one arm and hand in the paper bag.
Radiators do not "blow" fibers. The heat from a radiator does just that- radiates - with no air pushing it. The cast iron of the radiator itself heats the room. The Rs may have had forced air heat too, but believe me, the fibers would not "blow" THROUGH HER CLOTHES or INSIDE HER PANTIES. Doesn't happen.
The "dark" fibers could have had any one of a few sources. JR's shirt had been mentioned, but as they already tested it against fibers in the panties, I can't imagine they wouldn't have tried against other dark fibers as well. They knew it was wool, they knew Patsy's jacket was acrylic fleece. I believe some of the dark fibers were said to be cotton. There was a dark navy terry bathrobe belonging to JR found in the den- an odd place to be sure, for a bathrobe. There may have been dark towels in the house as well.
The white paper could have been something just stuck on the carpets somewhere. Not EVERY bit found on JB had something to do with the crime.
But for the fibers of the parents' clothing to be on the body of a child killed the day they wore those exact garments AND to be found on items like the duct tape, panties, garrote knot and paint tote, all items specific to the crime, puts to rest any claims that fibers from people who lived there would be expected to be there. Not on THOSE items, they wouldn't.