Ah right. I did a Wiggle sportive with closed roads but you'd just not trust road closures. Which paid off as a Landrover came around a blind bend in the lanes.
Would those roads get much traffic anyway? I'm sceptical of feed stations too. Great when they work but sometimes there's huge queues and or limited food / drink. Easier to take most of your own like you'd do on most rides.
If you give 'Etape Caledonia', 'farmers' and 'road closures' a Google you'll find a good few articles and pieces on the relationship between the public and the event. That said, the council and event organisers (and volunteer marshalls) strictly enforce the road closures and, for the most part, the public are accepting and adherent to the disruption between 0600-1200hrs on a Sunday.
In my 5(?) rides of the event i've never seen a vehicle or pedestrian on the route during the ride - 2021's unexplained open road ending aside.
I don't envision traffic would have been much of an issue on the route anyway, as you say they're quiet roads, but they are 100% the only roads. So if a Landrover comes belting around one of the many tight, nearly singletrack roads, your peloton will end up scattered into the surrounding fields.
Feed stations used to be good. Tea, coffee, hot food, bananas, flapjacks, sweets... again 2021 was the last one i did and it was as grim as trays of Tesco flapjacks and browning, nearly dead bananas. 2022 and 2023 may have improved it, but i don't have the experience.