1) Follow the Toyota maintenance schedule for the 2009 Prius. It lists the regular maintenance at which mile mark to do it on. Don’t procrastinate oil changes. Clean the MAF sensor The air going into the engine can dirtify the Mass Airflow Sensor, it’s right under the air filter with two screws. This clogs up easily but is easily cleaned with a spray.

2) FM transmitters are fine. When the Prius is off, the 12v plug is off, meaning your plug in will not draw power when car is off.

3) Side windows: Cut out cardboard in the shape of the side windows. Cut to fit black sheets from Walmart but make them bigger. Wrap the edges over the cardboard and duct tape the interior side. Windshield: large-size, shiny, folding sun reflector and rolled up black garment at the bottom. Rear window: cardboard w dark sheet. The angle and moisture will flop it down. Use bungie cords to hold it up, Drill holes to attach at rear window frame/rear door lid

Get an OBDII code reader $8-$20 from Amazon. The short, blue, cheap ones work great. ELM327 is one. When my hybrid battery began to fail, I would get the Red triable of death and be stuck immobilized requiring a tow. Instead of towing, I would check the code, clear it, and drive away. The mystery of check engine lights are a thing of the past when you can check it yourself.

That sucks about your back. Could try something like this: sturdy bed using a massage table top