Roommate's 2017 low coolant, unsure how to proceed.

See my post #2, #7, and #18. You gotta pay the mechanic for his time! If it makes you feel any better, that mechanic was an a-hole and that washer business is total crap-ola. If that drain plug wasn't leaking/dripping there's nothing to it. He was just looking for things to point at to justify what he was charging you. Why did he rip apart your CAT shield if there wasn't a leak???? Again, excuses to separate you from your money.....
I use the bare aluminum crush washers without issues or leaks. The OEM factory one doesn't have blue sealant on it. I know because I did a break-in oil change on mine @500 miles. Which I reused BTW. Eyeballed it a week later to check my work, because I reuse that old washer - No issues or drips.

 

No problem. Don't worry about so much about what others here at this forum write in posts, we all cross lines in communication or lack there of at times.
I had nearly the exact same issue 2 years ago and purchased a gallon of Toyota Super Long LIfe coolant from a Dealers parts dept. and put a couple of cups of it in the engines coolant reservoir. And I still don't know why it was low either. I'm just glad the coolant level has stayed the same since adding that couple of cups.
Keeping my fingers crossed that it remains that way.

Toyota Super Long Life Coolant is a 50/50 mix that is added directly to the cooling system.

Toyota Long Life Coolant is concentrated coolant that needs to be diluted with 50% distilled water before adding it to the cars coolant system.

 

Ive never had an issue with the coated Toyota oil pan drain bolt washers, but I DID with Honda’s: I’d torque to spec, and found at next oil change the drain bolt loosened way too easy, alarmingly so. I surmised the coating was too compressible or deteriorated somehow, asked at the dealership parts department and found they’d also sell me a plain aluminum washer.

yeah food for thought; just one more “improvement” that can backfire.

 

I would bet the mechanic was giving you a hard time because he realized that when they changed the coolant in July they messed up by not finishing the job correctly and thus the slightly low level. Basically he was gaslighting you (making you feel like you don’t know anything to cover his mistakes).

Take a deep breath and remind yourself that while you may be autistic, you are not stupid and are smart enough to ask questions. The mechanic is the one that has the actual problems with people skills because he doesn’t even recognize his lack of ability and empathy.