Soakers (in this example) are a pain to retrofit, but do-able. I'd fit soakers everywhere they should be fitted, but if you're not getting an issue anywhere else.. its for you to decide.
You could try applying Stormdry to the chim to counteract damp issues. It's expensive, but the best I've used.
Ventilation becomes quite technical when you get stuck into it. Quite a few factors can affect what measures are required.
Thinking out loud here, but...
I guess you could incorporate some kind of ventilation around(or underneath) the fascia boards. Maybe extend/double the drip battens &...
I think you can use foil backed plasterboard if you use tape with it.. Unsure about that though to be honest.
We would use a sub deck (11mm is sufficient) with a bitumen felt layer as vapour barrier. 100mm insulation over that & an 18mm OSB top deck screwed into roof joists with large screws...
Hard to tell from the pics but some possible problematic issues could be: (not that these are problems I can see.. Just suggestions to check)
By process of elimination, I'd check:
1. Are there any soakers between vertical tiling and original roof slope.
2. Lead flashings & soakers
3. Back...
I wouldn't install 100mm on that bay. I'd be more inclined to go with a cold roof & insulate between joists.. For the visual considerations.
The regs guidelines are to be achieved where possible, but exceptions are allowed where installing to the guidelines is not possible.
Stick with a warm...
Agree with Vinn. The more pitch the better, but def consider the flashings.
You'll probably have to alter the gauge if you're going to be close to the minimum pitch. Check the product specifications for low pitch installation.
Also, if you're installing roof Windows, you'll have to check the...