"Ms Serene"
Seren is anything but serene.

No ending e on her name. Seren is Welsh for star.
I'm German-Welsh, and she's a German breed, so I decided that she needed a Welsh name.
I was thinking Rhian or Rhiannon but I didn't want people saying "OH wow, you must be a huge Fleetwood Mac fan!"
Well, I am, but I didn't want people thinking I named my dog for the song.
Instead, I've had a number of people ask me why I named her after a German WW II nerve gas and am I a Nazi or something?
Jesus wept...

Anyway...
I just bought a hard bottom rear seat hammock to replace the sling that I had for years. It has made it much better for Seren as she doesn't get stuck in the swale. She'd sit in the swale (where the fabric dipped between the backs of the front seats and the rear seat base and she'd have trouble getting out of it.
The hard bottom seat extender has made a HUGE difference in her ability to safely and comfortably ride in the rear seat.
I'm not sure that the one I have supports partially folding down the rear seat. I'll have to look, but I'd have to load her in the rear, get her to move up to the seat, maneuver over the folded seat to the point where I could fold it back up.
As for keeping the rear seats down and keeping the whole rear area as her carrier... then where would I stow the ramp or stairs that I need to get her into or out of the Forester? I don't have roof rails on this Forester.
I can't have her AND the ramp or stairs all riding loose in the cargo area. That's a recipe for absolute disaster.
She's got to either ride in the rear seat or she's got to ride in the cargo area.
Ideally this ramp will work in getting her into the rear seat and I can then stow it in the cargo area. That would be the best solution. But I won't know until I see it.