Hearthpwn

Having a board dominance is essential here. Don't waste turn on Grimestreet Outfitter if opponent already has something on board and you can't trade it. Playing Smuggler's Run on turn 1 is also arguable as sometimes it is better to have body on board. You can also consider mulligan them away.

Wise trades are key to victory. Don't be greedy and sacrifice that Murloc Tidecaller if it gains you a board.

Discovers

Eye for an Eye - just forget it, I can't really imagine a situation when this could be more usefull than other secret against any Aggro deck (Mortal Strike or Kill Command to draw match maybe ... but what are the odds?)
Getaway Kodo - Great to have your Battlecry minion back. Bait opponent to trigger it by killing Grimestreet Outfitter or any Battlecry Murloc except for Coldlight Oracle (you don't really want your Aggro opponent to draw 2 cards)
Noble Sacrifice - Possibly the best secret against Aggro. Can really screw those Leeroys.
Redemption - Even better over Noble Sacrifice if you have Righteous Protector on board. Could also bring your Murlocs to life to buff Murloc Tidecaller, but yet not too synergized with a buff strategy of the deck.
Repentance - Mostly useless against Aggro, viable if you have strong feeling of Bittertide Hydra comming.

Stonehill Defender - If you have the choice pick Wickerflame Burnbristle - it is great against Aggro, even more with your handbuffs. Sunkeeper Tarim is also a great choice if you have board dominance. Tirion Fordring and The Lich King are fine, but I wouldn't pick them on turn 3 against aggro as it is really risky. If you have no legendary choice, then go just for a cheap high health minion.

Primalfin Lookout - Your primary discovery should be Coldlight Seer if you have 2 or more murlocs in game no matter that you can pick Murloc Warleader or Finja, the Flying Star. The reason is simple. You need health to make a good trades. Sure in case you need an attack you pick Murloc Warleader and if you need to fill the board you pick Finja, the Flying Star. Discovers are mainly about answers. You need 2 extra damage, well pick Bluegill Warrior or even Grimscale Oracle. You shouldn't always blindly pick the rarest drop.

Mulligan

Righteous Protector - Awlays
Murloc Tidecaller - Always
Rockpool Hunter - If you have 1-drop Murloc
Hydrologist - If you have any 1-drop
Smuggler's Run - If you don't have 1-drop, but you have 2-drop
Grimestreet Outfitter - If you have no other turn 2 play from hand
Grimscale Chum - If you have 1 or 2 mana Murloc
Coldlight Seer - If you have at least 2 Murlocs avaliable to play within first 2 turns
Primalfin Lookout - If you have at least 2 Murlocs avaliable to play within first 2 turns
Stonehill Defender - If you have turn 1 and turn 2 play from hand

Control decks usually run lot of removals so don't play your minions blindly as you would run out of steam in turn 7 if you do. Always prioritize buffs over drops and play around AOE and specific removals (Shadow Words, Mortal Coil, Volcanic Potion etc., ) and if you don't need to risk all-in, play around counter-boardspam like Spreading Plague or Mind Control Tech, if you do, you have really big chance to end it before turn 7 as tempo of this deck is really high for a midrange deck (it's more like Aggro Midrange deck though). In these matchups is essential to know your opponents deck, more than actual play. Just watch few streamers or youtube videos and you will get into it in no time. Always use Truesilver Champion for trading if you have a chance.

Discovers

Eye for an Eye - very situational, mostly against quest mage if you have suspicion that your opponent already have combo, or maybe even against Highlander Warlock to peel him more with his own AoE damage.
Getaway Kodo - Don't use it when you have bad minions on board as you can prepare for your opponent to ping something like Silverhand Recruit just to test your secret.
Noble Sacrifice - Good enough if your opponent is going on board with big threats
Redemption - Same as Getaway Kodo. Great secret but play it wisely
Repentance - Very powerful if you know what is comming. Especially against Highlander Warlock and Handlock with their dragons and giants.

Stonehill Defender - The best choice is possibly Tirion Fordring (only weakness is transform removals) and The Lich King (you will get value from him always, well, almost always, just dont play him into Potion of Polymorph though). Sunkeeper Tarim is fine too if you are loosing board control to try to comeback.

Primalfin Lookout - Mostly buffing murlocs, but you can also try to burn your opponent with Coldlight Oracle. It is pretty fun to watch Raza the Chained on fire :)

Mulligan

Murloc Tidecaller - Always
Smuggler's Run - Always
Hydrologist - Always
Grimestreet Outfitter - Always
Righteous Protector - Awlays
Rockpool Hunter - If you have 1-drop or 2-drop Murloc
Grimscale Chum - If you have Murloc
Coldlight Seer - If you have at least 2 Murlocs
Primalfin Lookout - If you have Murloc

Vilefin Inquisitor for Grimscale Chum
Dinosize for Frostwolf Warlord
Murloc Warleader for Coldlight Oracle and/or Primalfin Lookout
Gentle Megasaur for Frostwolf Warlord
Bittertide Hydra for Frostwolf Warlord
Sea Giant for Frostwolf Warlord
Wickerflame Burnbristle for Primalfin Lookout
Sunkeeper Tarim for Frostwolf Warlord
Tirion Fordring for Frostwolf Warlord
Genzo, the Shark for Coldlight Oracle
Finja, the Flying Star for Frostwolf Warlord
Leeroy Jenkins for Frostwolf Warlord
Don Han'Cho for Frostwolf Warlord
The Lich King for Frostwolf Warlord

If you want to add more than one High-end replacement and there is a conflict in the list, just let me know in comments bellow and I will try to help you. Doing a replacements in all combinations would be, you know, pretty impossible :)