Well, considering just about every PC has an ARM core somewhere (were they not used in many HDD controllers?), the legal requirement, that the system must have an ARM, would not be such a big problem for porting to x86. However I still believe, that it would make a lot more sense to make a cheap PCIe-card or Firewire-device with an ARM processor to run RISC OS on PC hardware.
I don't see point (c) as such a big problem, as long as Castle has to publish any changes they make to the open-source-components as well. IIRC that is what MySQL AB does.