Its going to be really interesting to see how the strength of this agenda evolves over time.

It will depends by the number of good neutral cards will be printed :3

I believe that ambush is a "put into play" effect, so no. Marshal refers to characters locations and attachments during setup, and play refers to events. 

"Marshal" refers to characters, locations and attachments entering play directly from your hand by paying their gold cost.

Using the Ambush keyword is a "put into play" effect. It is not considered "Marshaling" because a) you are using a card/keyword ability, not having the card enter play directly (i.e., without the use of a card ability), and b ) you pay the Ambush cost, not the card's own gold cost.

So, since Ambush is neither Marshaling nor playing (a term reserved for events only), Fealty cannot reduce the cost you pay for Ambush.

The key thing that you are missing is that the terms "Play" and "Put into Play" are two completely different things. For example, if you use Arianne's ability to put Melisandre into play from your hand, you cannot trigger Melisandre's ability because you did not play or marshal a R'hllor card, you put one into play. Something's cost for Ambush is completely separate from its regular cost and is unaffected by things that lower the cost to marshal or play.

Characters, locations and attachments are "Marshaled." Event cards are "Played." You get this from the RRG entries on "Marshal" and "Event Cards." Card abilities, including keywords, are "initiated" or "triggered." You get this from the RRG entry on "Abilities."

Further, the entry for "Enters Play" talks about marshaling or using a "put into play" card ability as two ways for a card to transition from an out of play state to an in play state. It does not talk about "play" being one of those ways (indeed, event card specifically never actually enter play). Thus, we know that "enters play" is the generic term for moving a card to an in-play state, not "play."

The word "play" is never used in the rules documents to refer to the initiation or triggering of a card ability, including keywords. It is only ever used in relation to using event cards. It is the conspicuous absence of the word "play" to mean such things as moving a card from out-of-play to in-play, or to mean initiating an ability, that lets us know there is a distinction.  

Thus, when we see the word "play" on cards like Fealty, we know it is referring to paying the gold cost of an event, not to paying any general cost that would ultimately move a card from out-of-play to in-play.