Yep, I've no real problem with RP being single source, despite what others in the hobby say it's not really different to other systems (including DCC) in that regard.
As to computer control, it depends on what you want to do with that. We have the ability to write programs for select RP hardware so rudimentary automation could be done now for accessories based on the hardware inputs received by the accessory module (there is no reason the hardware inputs couldn't be driven by another microcontroller device such an an Arduino either). Can't do that for locomotives yet though there shouldn't be any reason why it couldn't be done in the future. It might require new hardware like a detector that you write a program for that transmits commands to an LM that passes over it etc. The possibilities is there I'm sure it's just if TR want's to go down that road or not.
As to just using you computer as a controller, all you need is the CI-1 and to install HC Simulator onto the computer.
I don't think you can use multiple CI-1's and HC Sims at the same time.
You could, just as long as they have a computer each. HC Sim won't load more than one copy of itself into memory, and plugging two or more CI-1's into a single PC might confuse HC Sim if TR hasn't preempted someone doing that.
- Tim
