Alleged liquid damage on a 3 month Mac: is Apple pulling a fast one?

Hi! After limited use, my three month old Macbook Air suddenly started running very slowly with loud fan noises whilst I was uploading videos to Google Drive - I assumed this was due to it getting hot. As the issue continued, I tried resetting the SMC and the laptop wouldn't restart. I eventually managed to reset the SMC again and the computer turned on but again ran extremely slowly with the overactive fan. I took it into a Mac store and after examining it and they said there was a fault with the logic board, which they'd fix under warranty within a week or two.

However, 15 days later, they rang back and said they'd found corrosion on the flex cable caused by liquid damage and that some of the liquid indicators had been triggered. This is insane as I've not used the machine much and definitely definitely haven't had any spills. It's not been out of the house more than once and it's been in a case almost all of the time. The Genius I spoke to when I visited the store told me that as I couldn’t be with the machine 24 hours a day I couldn’t be 100% sure that it hadn’t been in contact with any water. As it is an inanimate object, I have no pets and the only person I live with is unable to lie, this made me question whether the same aspersions might be cast over Apple’s supposedly water-free repair room. My technical know-how is very limited, butI can only think that there might have been some issue with my computer in the first place that could have caused it to absorb liquid from a not very humid house, especially as I experienced some issues during the initial set up of the machine (it froze for a few hours).

They're now quoting £668 for a new top case, keyboard, trackpad, logic board and flex cable, plus labour. It seems crazy that I suddenly need all of these parts if just the flex cable is corroded: the Genius was unable to confirm whether the alleged liquid damage had caused damage to all of these parts and even told me that the cost would be less if I chose not to have them all replaced…

I would be extremely grateful for any advice!

My other concern is that the issue that caused the SMC to fail might be unconnected to the small amount of corrosion on the flex cable (please see image), but my something else. If I take the machine elsewhere and try replacing the cable, I’m concerned that the original problem might not go away and that I won’t have a leg to stand on with Apple.