If a Pokémon in a max raid battle were to set up a shield, would Unseen Fist bypass it? Will Unseen Fist bypass Max Guard?
To answer the first part of your question, Unseen Fist does not bypass a Max Raid Pokemon's Shield. I apologize for not having any proof, but in Sword, my Urshifu Multi Strike Form used Aerial Ace on a 5-Star Vikavolt. It was down to it's shield, and I hit it. It had the exact same effect as a normal move used, where it did a bit of Chip Damage and broke one part of the shield.
To answer the second part of your question, Unseen Fist does not bypass Max Guard. In the replay below, when Darkness Lariat is used on the Max Guarding Azumarill, it says "The opposing Also Not Top Snek [Azumarill] protected itself!"
From the official Pokemon website: Dark-type attacks used by Single Strike Style Gigantamax Urshifu will change to G-Max One Blow. Water-type attacks used by Rapid Strike Style Gigantamax Urshifu will change to G-Max Rapid Flow. G-Max One Blow and G-Max Rapid Flow can deal damage even if a target defends itself by using moves like Max Guard or Protect.
If you look at the ability on its own, contact moves can bypass all protecting moves, not including max guard.
Here is the page link (forgot to put it the first time and the second time):https://bulbapedia.bulbagarden.net/wiki/Unseen_Fist_(Ability)
According to this video, urshifu activated unseen fist with max knuckle, a contact move, and it did not pass through a raid boss shield. It only took out two shields, which is the norm for max moves.