I got it.
Not easy! But fun.
In case anyone wants them, here’s some hints for ending 3. Don’t read until you’ve solved A STUDY IN SQUID once through.
The lamp by the aquarium is ultra violet. There’s something it can read of Marcus’.
Use the typewriter in Marcus’ room (west of the entrance). It will spit out what looks like a blank piece of paper.
Read the blank paper in the aquarium with the lamp on. It describes a treasure hidden in the house.
But you can’t find treasure anywhere. You think you can’t. Big hint next.
If you entered the house, you have the treasure.
It’s the doorknob. Full solution next, eeesh.
After reading the blank page in the ultraviolet lamp by the aquarium, leave the house. Do not visit Marcus as he will have been murdered and you’ll be captured. Instead, just leave the house, and find the pawn shop. Pawn the doorknob. Go to Ruby. Propose. Then go to Marcus’ room and find his corpse. After being captured, you can escape into the manhole, ride the cart to gain ground on the goons, then get to Ruby and be safe.