Took a moment to establish what you're depicting in the diagram - we're looking at it top down, plan view right? I.e. the question is "can corner braces go on ceiling supports"

Are you using standoffs (post base connections that raise the timber above the ground)? End sealer doesn't give enough protection on its own against timber sitting directly in contact with the rot driving ground

@Chris_engagingdata Above and not touching is the ideal, I have seen plenty of cables run just above the insulation though and it's fine. Goal is to just avoid the cables being completely surrounded by timber and insulation i.e. your setup where the cables run along the rafter tie (bottom chord of the truss), assuming you put the cables towards the top then that would be all good.

@Chris_engagingdata Nice that certainly looks capable of carrying your ceiling. If you want piece of mind I would suggest having an engineer run the calcs with your local regulations in mind, but I'd be very suprised if it was undersized.

@FreeMan Found a reference on the rockwool website by the way - it unfortunately points at the UK code, but here we go: If cables cannot be raised above the insulation, a ‘derating’ of 50% should be assumed.

@Chris_engagingdata what spacing are they on and presumably all of your rafters are trusses? (i.e. they all have a tie). Cabling wise that sounds fine.

@FreeMan That one was from a retrofit in AZ where they were required to put cabling through conduit as the insulation made it a fire hazard. By the sound of it, probably one of those things that varies by state. It's certainly the law here in the UK (i.e. all regions here) as well.

Disclaimer I'm British, but it's definitely against US code to have cables effectively in direct contact with insulation for any notable length - they need to go inside conduit. This is true even for low load circuits such as LED lighting, as it might be LED lighting now but something else/ higher load in the future. Generally easier to have your cabling running broadly above the insulation and then take the shortest route punching through it. Modern building practices would suggest a warm roof instead though.

One important note is rafter ties are not rated to carry a ceiling. It is likely you need to add additional timbers to support the drywall.

If the house above is well built and sealed, the lack of an intake would result in negative pressure in the basement and effectively suck more radon out of the walls. An intake is very important as you can make somewhat of a guarantee about its freshness, particularly if it's coming from a HVAC system.

If you have a garden hose, maybe try running it near the ridge of the roof such that water runs over the area where you see the stains inside, but fully down the roof. Someone else looking at the underside of the roof deck can see where it is getting through straight away, without needing to wait for rain and being fully in control of the water ingress.

@gnicko if you think someone can diy a suitable notch cut, feel free to add another answer :) By far the most common thing I've seen is the rim mounted to the side of the post with metalwork underneath it. L Notching the very top of the post for the roof itself would be recommended but is a different joint/ not the one the OP is asking about.

@TimPowell I re-read your original post and it seems like having an inset rim probably isn't a requirement for you so I've added in some extra info on the common approach and joint detail.

Yeah, hurricane zone would require strapping on a joint like that which would limit the finish options. I'd be suprised if anywhere had a continous timber requirement but everywhere is different - definitely agree with checking what is permitted.

@jay613 a close vote is a more appropriate way to signal to someone that maybe they should take a look in the instructions rather than a somewhat hostile non-answer (as it was originally, it's toned down a bit now). Thank you for helping people as much as you do, but a little bit of politeness in today's broken world goes a long way.