Just some quick background on some choices.

Tess generally is a finisher, unless you are desperate, and depending on the matchup you need to think about the spells you play.

VS control warrior, you should only pick draw cards if absolutely necessary as tess will replay those.  You do have some leeway as your one copy of scheme is to immediately cast on her.  This should buy you more than enough time to outlast a control warrior.  

VS heal priest, she is only worthwhile if you are picking good aoe / destroy cards, and toggwaggles scheme is much better used on something else.

(etc)

Greenskin is in there to help buff your quest weapon if needed in some situations, mainly in combination with blade flurry.  But he also essentially saves you two mana a turn or two later.

Myra seems crazy odd, but she is another refill card and can be used to disrupt some decks gameplans.  Playing against a board clear heavy deck?  Pick a sticky minion like mech welp.  Playing agaist a highlander, that 5/5 that summons the lowest cost card from both hands has pulled zephyrs out so many times, and since myra only has 2 health, you can also backstab her the turn she is played if needed.  I initially included her just as a meme, but she rapidly became a very high win rate card for me in this deck.

Espionage is mainly a dead card, but is another last ditch deck refill if your tess kills herself, or if you are playing a slow enough deck.

The main problem I have is when I draw horrid and don't get any of my quest progresison cards, but generally speaking your mulligan is keep ANY cards that generate other class cards, with the exception of Bazaar Mugger.  You only keep him if you also have a clever disguise as well, otherwise hard mulligan for quest progression (and maybe keep 1 vendetta against decks like priest).