I believe several members have had better luck making their erosion control kind of a two-step process. (Since there is no perfect solution.)
I believe they have seeded with something like annual ryegrass that comes up fast, even under tough conditions, combined with the seeding for the longer term cover plant. They then overseed the longer-term plant over the next year or two to turn the patchy ground cover into a solid cover.
I am NOT an expert, so those are just some rough ideas for you to research since you didn't get an actual expert in your thread yet.
