As I understand it since the changes to Avios being awarded on spend, Iberia Express flights, bought through Iberia Express with an I2 flight number are now eligible for Avios and Tier Points.
The Collecting Avios and Tier Points on flights page on BA.com, under Iberia, now says:
Iberia
At a glance
Included: Iberia (IB), Iberia Express (I2), Iberia Regional Air Nostrum.
Excluded: flights operated by Level on behalf of Iberia.
Collect Tier Points? Yes.
Add-ons: you can collect Avios for eligible add-ons you buy as part of your trip
That said, I am currently in a back and forth with BAEC to them to credit an I2 flight.
I always find point to point flights are cheaper when purchased through Iberia Express. You can add your BA Exec Club number in the booking process, but it won't recognise any oneworld benefits and will want to charge you for seat selection. Once booked use the Iberia Express locator to add the booking to the Iberia app which can then be used for seat selection and check-in. Don't use the Iberia Express website to check-in as you won't get any oneworld benefits show up on your boarding pass. Generating a boarding pass through the Iberia app will show Fast Track etc if you're entitled to that.