All player actions are triggered (in that the player chooses for them to happen), including paying gold as the active player to Marshal a card. However, not all player actions are "triggered effects" (eg, Marshaling the card or declaring the challenge from Blackwood or Shadow Politics).
As far as Slynt goes, Ambush is an effect, Marshaling a card is a mechanic. Whether Slynt would impact declaring the Blackwood/Shadow Politics challenge (ie, whether it is considered an effect or a mechanic) is a little muddier to me. I could argue it both ways. You might want to send that particular possibility into Damon.
Thanks for being thorough, thought Blackwood/Politics are irrelevant to Slynt as they do not have an influence cost. I was just trying to be sure I grasped what all constituted a Player Action. Taking a look at pp. 13-14 in the FAQ would have helped:
During the course of the game, players may take many different types of actions. For the purposes of these rules, an action is defined as any time a player plays a card (including marshalling), or chooses to use an ability on a card already in play, including responses. A passive ability is defined as an ability on a card already in play that triggers automatically, without a choice from the player. Actions and passive abilities are separate terms that are not interchangeable; if a card has a passive ability, using that ability is not considered taking an action.
To "take a player action" is to do one of the following:
1) Play a character, location, or attachment card from your hand (during the marshalling phase, and by the active player only).
2) Play an event card from your hand (this is also called "triggering" an event card ability).
3) Trigger a card effect printed on one of the character, location, or attachment cards you control in play (or, in rare circumstances, from out of play).
For the instances 2 and 3 above, the card text effect will always be preempted by the exact phase in which the action is allowed. Examples are Marshalling:, Challenges:, or Any Phase:.
I guess the solution for my question was in front of my nose. #3 indicates triggering an effect, the explanatory text mentions "Any Phase:" and the definition of Ambush mentions the same. All so neatly transitive. I tend to agree that things are some things are a bit slipshod, or opaque to the newbie, but in the end it all does make its own ***** sort of sense. I really do think most of my "complaints" (which is perhaps too strong, gnat-like nuisances would suffice) could be solved by a more thorough FAQ. And that could likely be handled in a curated wiki a la the ktome. Thanks for all your hard (and repetitive) work.