If you had a murder victim to hide, why might their clothing be removed? I can only think of three reasons
1. they already had all their clothes removed when they were killed
2. you want to wash their clothes to remove your traces
3. you want to lay a trail of clothes somewhere for 2 reasons - a/ to steer police away from where you hide the body and finding it, and b/ to make it appear to be a suicide - near cliff top.
If it's 3, you make up a story that they were so hot at your house that they started removing their clothes, and you convinced them to put their clothes back on, to lay the seeds for the eventual bizarre behaviour of the (hot) suicidal person stripping off first.
I wonder at what stage did RD first say to anyone that Gaia took off her top.
I wonder if person who had the clothes could not plant them during first week because there were searchers out and police monitoring them.
Does it seem strange that the clothing appeared on the same day that PE was found after being missing for 48 hours, or is that a massive coincidence? Could it have been the friend's car was borrowed to drive into Swanage undetected, under cover of darkness/early hours, and PE crept along the field, keeping close to the wall, and dropped them there or threw them over the wall to the next field?
It was surely a mistake to put the clothing in a place that had already been searched, but then again what choice did the perpetrator have, because if it was found say 10 miles away it rather rules out an impulsive suicide and the fact of Gaia not being spotted on any cameras leaving Swanage on that journey. It had to be a short route she would not have been spotted on - close to the E house.
Just some thoughts.