I'm starting with the assumption that booking Main Cabin on HA means that the seat selection rules only apply to HA and not Alaska. Since you're booking Main on HA, you have the ability to select seats on the HA flight, which I think you've already indicated you've done.

On Alaska, the key is the fare code. You can pre-select seats on Alaska for free (they will give you options in Economy for free, but you would have to pay an additional fee if you wanted to book Premium Economy or Exit row) - the only exception is if you booked a Saver fare. The Saver fare does not allow you to select seats until check-in. Look at the fare class code on the Alaska leg to see whether it is Saver or not, a Saver fare is booked in X class. Any other class like G, T, S, N, etc. would be okay and allow you to select seats as soon as you book the reservation.

When you book the ticket on Hawaiian, they will give you an AS confirmation code (Alaska is AS, not AA), and then you can go to the AS website or mobile app and select seats. If you get a worst case scenario, remember you can always cancel a flight within 24 hours and get your money back, allowing you to re-book. I hope that helps!