Good news. The car is fine! Compression test had come out within normal range at an average of 137Kpa. The rattling is from a cat going out. Luckily the car is still covered under the emissions warranty, as they told me without it, it’s a 3K job. Going to have them do a flush of all fluids (Except trans as they won’t do it). Thanks for everyone’s help!

 

That is great news! Thank you for the update! Now people who come across this thread will know what a bad catalytic converter sounds like.

 

Did they figure out why you're losing coolant?

 

No, not yet. I guess it could have evaporated off and a service advisor told me this is normal over time but ehhhh idk about that. It seemed to have dropped to the point it did in about a week or so.
My guess is either a pinhole in a radiator hose (which if so great! I now have an excuse to get the Z1 silicone hoses), the radiator itself, reservoir, or I didn't let the car warm up to a high enough temp. Maybe the radiator cap?
As far as I know worst-case scenario, now that I know it's not a head gasket issue, is the trans fluid and coolant are mixing.
I plan to have all fluids changed out tomorrow and will be sure to have them pressure test the system and check for any signs of trans fluid in the coolant just to be safe.