I've used that path a couple of times and it is absolutely fine. In fact (and I can't quite believe I am saying this) it is preferable to the road, though that's mainly due to the woefully poor surface of the road.
At the top of the hill, it's a n
arrow 1m wide rutted cycle lane with a 2m wide stretch of hatchings and then the lane for traffic. The condition of the cycle lane does improve as you drop down, but if you veer onto the hatchings, you get bounced around, and at speed it can be hard to stick within the lane. Realistically I expect they'll have their vehicles on the hatchings as a minimum and so would not be safe to use, especially as any diversion means a massive detour.
When we went down it last week, they'd only been resurfacing the traffic lane, but once it's finished, it will be lovely, as the main road is great for cycling on as it's rare to get any cars in the small hours.
Yes, we could use the separate lane on the other side, but I'd prefer not to, as it's not as good as being on the road, and I run enough Sustrans rides as it is!