p. 16 of the Rulebook: Whether an enemy is engaged through an engagement check, through a card effect, or through a player’s choice, the end result is the same, with the enemy and the player engaging one another. In all cases, the player is considered to have engaged the enemy and the enemy is considered to have engaged the player.
So yes, Hummerhorn will deal its 5 damage, whether you engage it optionaly or it engages you because of your threat. Same with Forest Spider: it'll receive its bonus independantly of who engaged who.
p. 25 of the Rulebook: When a player plays an attachment card, he has the option of giving control of that card to another player by attaching the card to one of that player’s characters. Players always assume control of attachments that have been played on their characters. If control of that character changes, so does the control of any attachments on that character.
So yes, you can play attachment on another player's cards; if you do so, that player will fully control that attachment (different than the way Aura work in Magic: the Gathering).
Allies are always played under your own control, but certain effects can later have them change control (see Wandering Took). If that happens, both the ally and any attachments on it will change control.
Finally, for events, you must refer to the text itself. If the card refers to "you", then you cannot play it on another player.