I think this is a neat concept, but I would probably lower the threshold to 2 cards during the turn rather than 3. The slight grammatical error has been mentioned, so I'll take a stab at the wording for how I might lay it out:

Friendly Inkeeper:
"When you become startled, you gain 2 life"

Sounds good. Sets precedent for "become(s) startled" as a keyword.

Doubt Self:
"Counter target spell unless its controller pays [2].
If that player is startled, counter target spell instead."

Sets precedent for "is startled" as a keyword. Added target to the second stipulation. I also think it should be two lines on the card, since it's two separate functions rather than one.

Rooftop Assassin
"Whenever Rooftop Assassin attacks, if the defending player is startled, it gains deathtouch until end of turn."

Works well as is.

Nightmaric Flames:
"Nightmaric Flames costs [2] less to cast if its owner is startled"
"Nightmaric Flames deals 3 damage to target creature or player"

I think instead of "you're startled" it should be "its owner is startled" in order to consolidate triggers to "becomes startled" and "is startled". It also eliminates the rare issue of an opponent somehow being able to cast your card after startling you for cheap. If you don't decide to make being startled only ever a punishment or only ever a reward, I think you could clean up the edges on cards like this one by offering a downside to balance the upside. Maybe having it deal 3 damage to target creature or player and 3 damage to you as well. That way being startled is always a punishment, but sometimes it balances itself out.

Creakwood Guardian
"Trample
Whenever you become startled, Creakwood Guardian gets +3/+3. until end of turn"

Again, changed it to "you become startled" rather than "you're startled". I also think this one should have a downside to balance the upside as well. Maybe it gets +3/-3 instead. That way there's incentive for your opponent to make you startled, by putting your creature into bolt range, but you also get a big swinger to make up for it.

Guardian Gargoyle "[T]: Add [1] to your mana pool.
Whenever you become startled, Guardian Gargoyle becomes a 2/3 Golem artifact creature until end of turn."

Changed to "become startled" rather than "you're startled". It's tough to imagine a downside to balance it out on this one. Maybe making it a flyer as well and having a stipulation of "As long as Guardian Gargoyle is a creature, creatures you control get -1/-0. I dunno. Maybe the negative effects have diminishing impact as rarity goes up.

All in all I think this could be really cool on the whole. The only problem I think you may run into as a set is making the engine run in a fun way. I imagine a set that is designed around cards going to the graveyard each turn would have to have some serious recursion aspects built in in order to not be a chore.