At Area 4, go underneath the area and use the left Inkrail. Then use the Geyser and jump to the center platform leading to the boss kettle. There is an Inkrail leading to Mission 20. It takes place in a modified version of the multiplayer stage Bluefin Depot.
For the Sunken Scroll, at the final checkpoint under the grated ramp just ahead of the Inkrail. Get to the far Northern part of the map from spawn. Directly in front of the Inkrail leading up to the Zapfish is a grate ramp. Go through it in squid form and find the scroll at the bottom.