This deck is a go second deck with a lot of tool box cards. Magical Musketeer has been a tier 3 and lower deck due to the sheer amount of power creep in the last decade. Its only strength is the spell/trap handtraps that interact with the opponent by negating cards, destroying face up cards, and banishing cards from grave. Its best matchup in the current format is stun since you can pick off fossil dyna more consistently than they can open judgment. (A testament of how slow a deck needs to be in order for the deck to dominate).
The Fiendsmith support provides the consistency that Musketeer has longed for because you can now search Caspar, Starfire or whatever Musketeer you need. Basically, you are now running 9 copies of Caspar/Starfire and 7 copies of Doc, Calamity, Wild and Kidbrave without taking up the deck space. You could run a similar Small World Package consisting of Ash, Effect Veiler and Nibiru as Small World bridges, but you always had to a) draw no Musketeers and b) needed one of the aforementioned drawn with Small World for the package to be worth it. Fiendsmith works as both a way to get to Starfire/Caspar and as a way to proc them. Its just an efficient searcher for Musketeers. Also, fi you are set on Musketeers, Lurrie is searched and dumped to become the second body needed to make Sequentia.
The Fiendsmith combo lets your go first endboard be more than Caspar + Starfire pass, converting into Apollousa or High King Caesar with a Musketeer on board that can't be targeted. Prior to Fiendsmith, if you didn't open Caspar or Starfire going first, you were in a bad spot. Going second, if Max got negated, you just passed, now you can convert it into Fiendsmith stuff, making cards like Ash only stop the Musketeer side of the deck, requiring 2 negations minimum to stop the entire combo. Overall, the Fiendsmith engine is a welcome addition and should be run moving forward in every Musketeer deck.
As for the the power cards of the deck, DRNM, Super Poly, and Link into the Vrains all have the clause: your opponent cannot respond, making them 'guaranteed' against most meta decks. You side between them for the appropriate matchup. Evenly and backrow removal is to hit back row decks, but Evenly hits all but 1 card in every matchup, so it is in main for that reason. The idea is to ensure your Muskets can hit the field and these cards generate advantage through denying negation. Super Poly eats a minimum of 2 cards, DRNM negates all the monsters currently on field, and Link into the Vrains guarantees Max resolving his mass search effect or summon effect. If any of light fiend links get destroyed, it recurs 1 light fiend from grave (per copy of LITV). Also, it serves as an extender, so if you get Nibbed, you can just normal summon to continue your plays.
Extra deck is running the Musketeer and Fiendsmith combo. Aside from named cards, the rest is just tool box. High King Caesar and Apollousa are just good generic cards to stop your opponent from playing in ways that Muskets can't stop. You can toss in S:P in as well, but it is by no means required. Super poly targets are flexible, but Garura and Mudragon hit most decks in existence with the other 2 being tailored to whatever is currently meta. Also In the mirror, Fiend Smith is a free summon for Lacrimosa and Dies Irae.
Side deck can be tailered to the event you are going to, so this is by no means fixed. Run more s/t removal if your locals is all stun, run more anti Snake-eyes (or whatever is tier 1) cards if you are going to a YCS. Make adjustments to have a better matchup against the most popular decks.
Final Notes:
Runick Fiendsmith Musket is a build I was contemplating but after thorough testing with Runick Muskets in TCG and MD, there are far too many weaknesses to warrant running it with Fiendsmith Musket. You cut Musket S/t to 1 make space for Runick cards, the power cards are (unsurprisingly) more powerful than any of the Runicks, the draw 3 effect of Fountain requires you to draw into Runicks to keep that engine going, and the no BP clause makes the game artifically longer. If Fountain never goes up OR if it goes down, Runick stops working on your opponent's turn. Fiendsmith puts up a lot of damage on board on behalf of the otherwise weak Musketeers, so you can close out games quicker with the non Runick variant. Quicker games, means you grind less in every game, and mitigate the risk of going into time. Also longer games take its toll on you as a player for 9+ round events.
In the midst of a potential ftk format (via Gimmick Puppet), you do need to run handtraps like Imperm to stop as much of the burn as possible when you can't get a Musket on field. However, when there is just build a board meta that end on negates, you can swap out the imperms for more copies of Musket cards since that will increase the likelihood of drawing them and putting far less strain on Caspar and Max to search them. The main reason handtraps are so effective against this deck is because they prevent you from seeing your S/Ts, but if you hard draw them, the handtraps thrown at you do not matter/ matter far less than if you ran 1 ofs. That being said, running 1 copy of each S/T is 'wrong' in the sense that you must resolve your effects in the face of raining handtraps to get to your S/Ts.
One card I may consider is Spooky Dogwood since ftks and tenpai are expected to be prevalent when this archetype is released, so these just turn off their ability to kill you outright. and give you a turn to establish board and take over the duel from there. If that doesn't do the job, then Dimension Barrier hits both decks pretty hard.