Sifting through the threads I can't make heads or tails of it. From my understanding the shield spell can somehow trigger from a spell that uses an attack roll (the spell being considered an "attack" in this circumstance) yet for things like extra attack, spells are not considered an "attack." Personally I would argue that shield spell can't trigger from a spell attack but instead only from a melee or ranged weapon attack and the obvious magic missile as well.
The actions Attack and Cast a Spell are separate action options. You either use one or the other. Extra Attack requires you to use the Attack action:
Beginning at 5th leveI, you can attack twice, instead of once, whenever you take the Attack action on your turn. (PHB 72)
Thus you cannot cast two spells with Extra Attack. However, both are capable of launching an attack (note the lowercase 'a'), which is anything that requires an attack roll. The shield spell can be used against any kind of attack, not just ones launched with the Attack action, as it states no such requirement.