Improve accuracy of Product Backlog estimates and prioritize items properly
Don’t avoid grooming meetings and discuss you backlog with stakeholders as much as you can. Make sure that people understand you. And one more thing – you must really want that a certain item/vision/idea would be done.
Fixed scope agile
I believe it is possible, but sales people (or probably even customer) must be ready for that. I refer to this great article where samples and descriptions are provided - Agile + Contracts.
I came to a personal conclusion that you shouldn’t compare these methods. And there is only reason for that. All methods are great! But when human comes into process adoption, everything can be distorted. Use what suits you best.