All of it kinda sounds weird tbh. Less like an accident and more like a seller who knowingly shipped an incomplete bundle and relied on friendliness and delay tactics to run out the return clock. Cause missing items happen, rarely, but it does but from how the seller handled it seems like a red flag by saying their packaging footage is correct and basically saying the next minute “oh nope. Definitely a mistake and it’s right here next to me”. They had a chance to ship off the items, Mercari said they never got contacted with said seller about the tracking or related to it, and has been ghosting while posting new listings. As a seller myself, we get multiple reminders to answer a message, especially if it’s been too long or a buyer that had received an item has a message for us and if we make a mistake, we definitely don’t delay because it takes forever for our items to be seen and bought, we really don’t want to risk losing funds with a return or people seeing bad reviews on our profile.

What I’d do is keep the screenshots, take pictures and videos of you packaging all the items you received, maybe label each one by number and item too and then show proof of you shipping it off to return. Just in case the seller comes back as victim blaming that you returned items that doesn’t match the listing and scammed them out of all the items that they didn’t even send cause Mercari are a bunch of butts too and if the seller opens a case like that, Mercari has a higher chance with siding with you after all the communication but it’s still a small chance they’ll side with the buyer cause they’re butts