@BoomfrogSlow
A creature checks whether or not it can take an additional action at the moment it would declare the action. If it acquires the Slow trait before it takes the action, it may not be able to take the action.
Giving the creature slow after they take a move action does prevent them from taking an additional action.
@WildhornThere have been extensive discussions about this before, but currently the Official Rules and Codex Supplement says the following:
Magebane
Magebane deals its damage after the spell that triggered it has been resolved. For example, when this creature casts a spell, you could reveal this at the end of the Counter Spell Step, and then at the end of Resolve Spell Step, after the spell resolves, this creature will take 1 direct damage.
Thus, currently, revealing it after the Counter Spell Step does cause the creature to take damage.
@BiblofilterYou could use Enchantment Transfusion before damage is Applied in step 4 to move the Healing Charm to a safe target, yes. Alternatively, you could use Enchantment transfusion before damage is applied to move Bull's Endurance and save Sosruko.
If a Non-flying creature attacks, and you reveal Eagle Wings, your creature will
NOT gain flying until the attack has ended. If you read the rules on page 15 that are referenced in your quote, it specifies that a creature can not gain flying during an attack as well as being unable to lose it. Thus, you will not avoid the attack, and it will not be cancelled.
The flying trick only works if you reveal Eagle Wings before they declare their attack. Creature actions basically work as follows:
1) Activate the Creature
2) You may move the creature
3) You may take an action, but you can only take a full action if you did not move.
Therefore, you could reveal it after they activate the creature but before they do anything with it. You could also reveal it after they move into your zone but before they declare an attack. Both of those options would result in the trickiness you desire. However, as soon as they declare their attack, Eagle Wings will have no affect on it. Thus, this trick will obviously never work against a Flying Creature.
If an attack is cancelled for a different reason though, you are correct in that they do not get their action back. Their action is simply forfeit.