You can't have different fare basis or fare buckets in a single booking, whether you telephone or online, so that's not going to work. You will have to make two separate bookings if you are to be on economy plus and your wife on economy basic.

Your Silver baggage or free seat selection benefits won't apply to your wife's booking if you make two separate bookings.

Using Avios to discount a paid ticket will earn you Avios and TP.