Cliffnotes for a walkthrough:
Mission 1
Grab 2 Plates from left and right path branches, insert them to circuit box next to the locked door, pass through.
Sewer key is on a pallet next to the wall to the right when entering.
Unlock door, climb, approach room exit to trigger 3 minute arena.
You can totally mow down all of the Zombies if you can score headshots - they drop ammo after all. Just be careful, combat loot disappears after a short while. Otherwise, just move in a circle around the room, vault the obstacles, they can't really keep up with you.
Climb up into open area. Press M to enable a target beacon for the mansion.
Mansion gate is locked, Zombies right next to the brick walls drop it. (Left corner when looking at gate towards mansion)
Mansion front door is blocked by cabinet, go to the side and push to clear. Key for front door is on a shelf all the way to the left on the balcony.
Now inside mansion. Meet first fat guy. If you are clothed, he will do an unavoidable(?) charge attack that strips you. Just stay stripped and they won't do the charge attack. Don't get grabbed, i dont think you can break free and its instadeath. Sprinting past at a minimal distance seems safe enough. I dont think they can die.
The goal is to collect 4 plates and put them into the machine on the wall to the right. There is several doors and paths, but since they are locked by subsequent keys (heart, club, spade, diamond) it's fairly linear.
When entering, the first open paths are through a door in the back right (with a short hallway from main hall) and to the right. Back right door leads to heart door and a stairwell to second level. Heart key is on a cabinet on second level. Plate and key in heart room.
Exit back to main hall. Use other open door. At the end of a long hall the door for the key you found in heart room. Stairwell to level two, follow it to get to main hall second floor. You have to push some cabinets to get at stuff under them. Hall half-hidden behind shelf opposite to your entrance point. Plate in cabinet, next key near the end of the halfhidden hall. Beware fat guy.
With next key, go back all the way to main hall floor. Go through the first door again. Key opens door at end of long hallway on second floor. Plate and key inside i think?
Last key diamond opens door from main hall, left side. Final plate through there. Put all plates in machine and touch the green crystal to "end the mission".
I might have the key/door order wrong, but the instructions themselves should be good.
Mission 2
Go to mansion. Front gate is unlocked, front door key is on the inside of the front gate.
Basically re-do a mansion run except the plates and keys have shifted positions. Notes: One plate is now half-hiding behind a shelf on the main hall second floor. Heart key is in the open near the heart door. Diamond door is optional, there is no plate inside.
Once all plates are placed, exit the mansion and go back to the sewer grate. Enter.
As you backtrack, kill all enemies because they drop a plate for the breaker box and the sewer key.
The second plate is stuck to the ceiling near the green goal, shoot it to get it. The third plate is in the room with the dogs, "islands and bridges", on a barrel. In mission 1 there was ammo in that spot. The final plate is at the Mission 1 spawning point.
The breaker box is on the wall in the room with the dogs and "islands with bridges", basically the first big room from Mission 1. Return to goal in the arena room and touch it to finish this version.
Beating mission 1 unlocks some hairstyles and the secret outfit. Beating mission 2 unlocks nothing, as far i can tell.