You would check by movement if moving is the only action they take, checking for traps, planning, even fighting, and any other action will take time, and time is what will set the roll.

Check this post for confirmation
Basically it's not on the rules.

When in doubt...

I'm not seeing other drawings in the same style, so it could be a public domain pic, but a google image search didn't showed anything...

I think the Eel is from Alexander Cook

Each character or creature involved in combat may move, if desired, up to its encounter movement distance, and then attack, if any opponent is in range, when its Initiative number comes up. After attacking, a character or creature may not move again until the next round Each character or creature i...

The Indigo Dragon also uses Illusionists spells... don't know if using another supplement is the best way to go (normally those spells should be added to the FGO to avoid having to add more supplements) Its whole backstory should be changed, it seems very setting specific, specially with the mention...

Absolutely, giving casters spontaneous casting + mana + all spell access is a huge overpowering, may work for some game styles but it will require lots of changes in other classes and areas. I was thinking of rolling a 1d4 for each level like hit points, adding constitution bonuses (or minuses) and ...

Yes, I made it.

Here is one option, other than that is as Solo said