Nobody knows yet which services will be moved to EZE, but if you buy SCL-AEP-IGR on the same ticket, LATAM should cover you in case any of the flights are moved to Ezeiza.

All Austral Embraer 190 have a 2-2 layout with average legroom (1-2 in Club Economy), and all of their planes but 2 have PTVs. IGR-SLA is ca. 1 hour, so you will get a soft drink and perhaps a tiny snack.

AFAIK Aerolineas' website allows for seat selection upon buying your tickets irrespective of your fare, so you can pick your seats for the whole family. Besides the 15kg baggage limit, bear in mind the Embraer carry-on luggage allowance is just 8 kg and in my experience they are very strict in that regard.

All AU flights are marketed as Aerolíneas Argentinas' ("AR2xxx opb Austral"), so you can earn miles on any Skyteam programme.