Flamewood and eater of world, other things.

I was wondering why flamewood isn't proc'd by eater of worlds or any "spell Attack by bosses.

Also why it isn't affected by several sources of added projectiles.

It is affected by everything you said.

Increased projectiles increases the damaging areas produced, allowing for overlapping damage. Check whether the source of increased projectiles has the "cannot support triggered skills" mod, because Avenging Flame is a triggered skill.

The avenging flame is triggered on hit, so whatever you are observing either isn't a "hit" or it misses your totem. It also has a fairly long cooldown (1 second), so you aren't going to be seeing raining fireballs anytime soon.

Eater of worlds is obviously hitting it, its visibly taking damage, and it isn't firing off.

I can't speak to your specific instance since I haven't tried it myself...but there are youtube videos showing Flamewood builds fighting Eater and having the totems cast their mortars...

Other eater spells does proc it - but default tentacles doesn't, i hope they make it like orb of storms at least - so it would proc by itself occasionally and with the trigger

i shred Eater with Flamewood. he doesnt like to attack the totems tho, you have to stand on top of the totems and when he basic attacks move or just tank it and they will hit the totem. same with 90% of bosses, also multiple projectiles does affect it, but it is not an ATTACK SKILL. so "attack skills have an additional projectile" does nothing because again its not an attack

Edit: slap on a sire of shards staff and watch the projectiles fly