Do monster attacks work the same way as PC attacks, so that by default the monsters add a 1d (easy) purple challenge die, plus any PC defence dice against the monsters attack?

Correct. Monsters can also spend out of their Aggression budget to add black dice representing maneuvers and other actions, and may have basic Dodge.

I find it's easy to forget this and have monsters/NPCs be defeated before they get to use those dice defensively or offensively.

If I monster has an expertise/specialisation trained can this be used to add a purple to the players roll or is it just blacks?

As I understand it - no. You only do that if you make an opposed test. Otherwise creatures have their Defence value and may use their dice resource to protect themselves. Rare creatures do also have Active Defence cards.

Re defense cards, I believe all creatures are assumed to have all basic defences they qualify for (e.g., animals are usually dodge only as they are not parrying or blocking)

The ACE budget can be spent representing fancier defensive moves etc., though I often also do things like goblins attack at night with lighting penalties etc.