re: So how far in advance was this whole thing planned?

The latter. This wasn't a great target honestly. Every cop is out and on high alert. It's not exactly the ideal day to go a-cop-hunting.

It turned out well for them, but I don't see how.

This was passion and opportunity.

Calculating would be something like picking off one or two and waiting for the parade funeral, taking a spot, and "executing" as many police as possible with a reduced chance of casualties.