Let's say I'm Preston. What stops me from using this card to exile lodge debts? I cannot choose to discard a weakness card, but don't see any prohibitions against exile in either the exile or weakness sections of the rules...

This is a very situational card. Usually, it's a testless clue that costs you a horror, an action, and a resource. Which isn't all that efficient a way to gain clues, unless you have some way to lessen the costs. However, if you have some way to make the costs less severe, it's a useful card for any investigator who doesn't have great investigating skills. The best use of it I've seen was a Yorick deck where he had ways of dealing with horror (Meat Cleaver and Cherished Keepsakes being the primary ones), and with his ability, he could put the raven into play without taking an action.

Yeah this card is Deck filler, If you don't understand what it does, it's Steadfast and its just as mediocre. To be fair to Steadfast and Curiosity, it'll never hurt you to use it, the effect might be minuscule but every little helps. The problem is that there's better options like Unexpected Courage, Perception, Guts, And a whole lot of other Seeker Skill cards. Its not bad, Not good, just Okay. Whenever I got it I just used it instantly or just forgot about it until the Scenario as it rots away in my deck. I would recommend ditching it and getting better cards. Also the image looks horrible, it looks like a rejected KISS member. However this card has the best flavor text in the game. "What if you are the cat?" Beautiful

What is the secret of Lily’s Discipline was in front of our eyes the whole time?

Not through the main attack that does only 1 damage, but the empowered attack that does 3 damage in one attack (same as for the Butterfly Swords (5)).

So if you use Bandolier (2) to get 2 copies down, you can Enchant both Weapons and the beauty of it is that if you exhaust one pair and then the other, that would count as 2 attacks for your Discipline.

Use your Enchant Weapon boost on your first attack to give it all the chances to land and trigger the second one.

This requires a bit of set-up and at least 5 cards, but this way you will be able to deal 8 damages reliably in 2 actions, without any lvl 5 weapon.

Keep Galvanize in hand for an extra burst, or your Fight Events like Spectral Razor, Toe to Toe or Sweeping Kick.

A nice deck for this version of Lily could look like this: arkhamdb.com and evolve with the Side Deck for a lot of stat boosts making her compatible with Hard/Expert

Updated to include The Scarlet Keys Player Expansion cards:

Directly:

• Ever Vigilant (and now Ever Vigilant lvl 4)
• Prepared for the Worst [lvl 2]
• Sleight of Hand
• "You owe me one!"
• Untimely Transaction
• Friends in Low Places (after taking the Swift upgrade).
• "I'll take that!"
• Salvage

Indirectly:

• Swift Reflexes
• Honed Instinct
• Dexter's ability.
• Joey "The Rat" Vigil.

There, someone should have catalogued Old Shotgun's enablers ages ago. Thank you to Until the End of Time for finding all of them.

Edit: Added Joey as per Django and Yenreb's insights. And I'll elaborate on how Dexter and Joey's fast playing of items allows Old Shotgun to enter play with ammo:

Step 1.) Play an event with a skill test (Backstab, Ríastrad, Clean Them Out, etc...).

Step 2.) During the skill test while resolving this event, you get a window of opportunity. Use this window of opportunity to trigger a fast play action with either Dexter or Joey.

Step 3.) You have technically played Old Shotgun while playing an event. Thus, is comes into play with its uses value as 2. Voila.