I've been working on slate roofs for 30 years and have never seen one that could not be walked on, unless it's to steep, which is often the case, but they still have to be able to support significant weight.
We would typically prop a ladder against the edge of the roof, and push a hook ladder up over the peak and climb up. That would be on a 2 storey and up structure. For a single storey, just lay the ladder flat on the roof, you have to bring the bottom of the ladder way back and stake it, it's a little funky going up and down a ladder in that position, but it's doable. you could always put another ladder next to that one and just use the ladder on the roof, above the edge.
IMO, you are overthinking this. it shouldn't be that diffecult.
Just be careful.
JB.
A couple of pics of some slate roofs for those who never get to see them.
These are monster roofs, we do use boom lifts to gain access. The gutters on the brownstone church are 55 feet off the ground, the ridge is 95 feet.