Does the Model 3 have an accessory mode for passengers?

[SMAlset]

Exactly. My wife’s phone is registered as a key for my Model 3 (even though she has yet to drive it...). I created a separate Tesla account for her and Tesla support linked her separate account to my Model 3. I’ll run into a store real quick while she waits in the car and she has no problems. Music keeps playing, A/C still running because the Model 3 still sees her phone in the car and doesn’t shut off. This is clearly the best option in your situation.

This is pretty much our situation. We are a two Tesla family now and each of us has our own account with the other having been given access to the other’s car thru our phone apps. Tesla sets it up. Think it took a day or two to process. When we just had my husband’s MS, I created a Tesla account, downloaded the app to my phone and he called Tesla to initiate the process. They had him send an email basically saying he was giving me permission to the car and identifying me (forgot all details requested). BTW this isn’t sharing one’s account (like buying accessories etc with owner’s registered credit card info or viewing docs), just ability to drive the car and guess in OP’s situation the ability to control accessories in the car, lock and unlock, etc. When I posted above forgot we had taken this step.

But if you or the wife have no interest in “sharing” the car with account permission, shouldn’t her phone set up as a key and/or giving her a card as a key (fob too) allow her the same basic access while she was waiting in the car. Just touch the screen from the passenger side to reactivate. Valet mode would be similar but with a few restrictions.

 

[SMAlset]

Isn't there a 'keep climate on' mode?

Yes there is and it is the simpliest way for a driver to provide a comfortable environment for a passenger left waiting in the car. I was looking through my camera photos today for something and saw that I took a photo of the update screens after I got 2019.5.15 on March 10 (had previously been on 2019.5.4 and not sure when I got that version). Anyway notice the section from previous release notes "Keep Climate On". The information on this feature was later added to the Model 3 Manual on 12/20/18 (page 101).

 

[SMAlset]

And yes, the audio system continues to be accessible when Keep Climate On is active. TeslaCam is still on and you can save a file.

However when set On, you CAN’T alter settings (fan speed, temperature, and some other settings like which vents are used). You CAN turn it off from the screen and adjust the air pathway on the display making the flow go up, down, split, etc. You also CAN charge your phone. Makes me think this was the Camper Mode that was talked about.