Hi Everyone,
I've tried a search but surprisingly wasn't able to find the answer to my question - I apologise if I missed it and just chose the wrong terms.

I've booked two Chile domestic flights on LATAM in "Light" fares. I booked on the Chile website as price difference was huge (footnote as I know this is heavily debated elsewhere, I found that it wouldn't allow me to at some hours but did at others without issue!)

I don't have status on Latam (and the airlines I have status with now don't make it valuable on Latam!) and so I realised that the Light fare only includes the cabin suitcase but not early seat selection (I first thought it was as "package no2" on Sky and Jetsmart includes seat assignment but then saw it isn't the case with Latam) and I don't particularly want to pay for seats on 2 hour long flights so if I understand correctly, will automatically receive the updated BP with the "random" seat assigned 48 hours before. Can I check if, after this happens, I am able to change seats, either at OLCI or at the airport (kiosk or counter?)? And if so, is it for free or for a fee? Any particular tip (apart from paying) on ideally avoiding the worst middle seats?

I've also seen with some level of worry that the max size for the cabin luggage on Latam is 55x35x25. Whilst height and depth look standard, 35cm is much narrower than any European airline (even Ryanair goes to 40cm and everyone else to 45!) so I actually don't believe that I have a suitcase that narrow. In existing threads, I've seen every possible version of how strict LA are with luggage size, from people reporting their experience of it never being checked to those saying it is very frequent - notably in Chile, using the sizers and inbuilt scales. Can anyone update me on the airline's "mood" at the moment? Flights are from SCL and CJC.

Thanks in advance!