Thanks for all the info...
The multiple application route does sound doable with some FA API work (which we'll have anyway) and backend API work on our end. We are looking at an application per customer already, which this would then multiply by the number of 3rd party services we take on, but that's probably not an issue (at this point at least).
We're a pretty small team (4 developers) that's vastly outnumbered by salespeople. We'll definitely keep the pro services in mind, but I think there is enough for us to get started with. I have a feeling we'll be moving to the Developer service pretty quickly after launch though. As the LDAP integration and Reactor service is very appealing.
Honestly, thanks for your thoughts and time on this..