Hi,
What a long thread and timespan since it was started… understandably, as all the questions posed are fair, worth answering properly and it looks like there are some interesting interpretations of the facts.
I will share with you some research I have done which lead to the direction I took. The link below takes you to the Homebridge server plugin I have setup on a Debian Linux - Hyper-V VM on my homeserver. It implements the Homebridge-Hue plugin which effectively maximises the accessories I am able to control via my Hue Bridge (Square One) syncing back to HomeKit for control on my IPhone via the Home App/Siri.
Whilst the direction (ie. Homebridge >> Homekit) is probably not for you, have a look at the Wiki docs on the site regardless, as the analysis shows the capability of the Hue Bridge and the workings of the supported protocols are clarified.
I hope this helps someone, sooner ask than assume.