worst of all - if your city starts building something, you can't change it until it's complete
Erm, you definitely can change what a city is currently building. I'm thinking about how to do it, and I'm coming up blank on how you would find it confusing, there are several intuitive ways to do it, two of which are:
1) Click City, click 'Contrruction', click what you want to build.
2) On the City's bar (over the city itself, not the UI overlay), click the cogwheel at the right hand side; this will bring up the construction menu.