Regarding the other stuff you mention; I have no idea why modifying placement.info would screw up anything to do with routing; they're completely unrelated, unless you're using Url Alternates in which case there is a very tenuous connection. Perhaps you
can give a specific example of what you're changing that's messing things up?
The thing to understand is, Placement.info will only affect shapes emitted from ContentPartDrivers. This is because placement is performed partly by the DriverResult, and partly by the content driver coordinator.
I've actually spent a bit of time writing a system that allows you to use Placement.info in other situations. It basically replicates a lot of what the drivers system does, but for other arbitrary models (not just content items). It's been quite interesting
to play around with. I'm using it for various composable UI scenarios in Science Project and Media Garden. Currently it's only available on a fork of Science Project but it'll be releasable soon.