What are we suppose to do for sewage?

Get creative ;) Sewage is intentionally a problem with this mission, to make it real challenging.

It seems odd that the map has two cargo terminals set up, with four branches of train tracks, but one of the branches is almost entirely inaccessible and the other is also somewhat poorly placed - and despite all the preexisting infrastructure, no passenger terminal is present.

The lack of population was a bit confusing until I remembered that people had to have a way to move into the city and thought of building such a terminal. Even though I restarted and did so immediately (creating a fifth branch of track in order to place it somewhere appealing, on that small road to the left of the tracks), basically every industrial and commercial building went abandoned due to lack of workers before I could get anything more than 10-30 people into the city, no matter how much high or low density residential space I set up.

As an aside, I caught this rather ironic chirp during my first attempt playing this map: http://steamcommunity.com/sharedfiles/filedetails/?id=423231905

Tallinu, this is all expected, since my plan here was to push players to fall into some fairly bad situations even before really starting. All part of the challenge :)

I find an airport is a more effective way to accumulate population, although it's expensive... (costs 15% of the starting budget)

I agree with you, winnerrmad. But that would also mean starting at a very high milestone, which could kill some of the fun/challenge.