Hi just been reading through the posts. Not sure if this helps much, but thought I'd add it none the less.
My wifes WRXs has what sounds like a similar squeak coming from the nearside, it basically seems to be where the dash meets the A pillar. If you place some force between the dash and pillar its stops. Though it is very annoying, given we know what it is, ie not mechanical, we've not done anything about it. I guess we should take it in and get it sorted.
As for the parcelshelf, yes ours rattles too, however we had a Forester sometime before this car and it was the same. I found if you cut up a suitable piece of sponge and pad the slot out a little it stops. It is crude, but works! just need to find some dark sponge.