Hi Sascha
Overall it sounds to me like you have a pretty strong approach.
In my experience the methods used to validate use cases is much less important than the methods used to generate them in the first place. The key to success is that your use cases (scenarios/user stories) have been derived from solid user research, align to strategic business objectives and account for technical constraints.
The validation workshop with SMEs sounds like a really good method to make sure that the project is heading in the right direction and should ensure that key stakeholders are engaged and informed. If you are defining and communicating a UX strategy and sharing research insight, this would be the opportunity to get buy-in.
I'd make sure that the SMEs you invite are truly representative and include individuals from all stakeholder groups (end users, project team, technical staff, business sponsors, etc). The trick is to include the right people at the right time and to ensure that the sessions do not turn into 'design by committee'. It also helps to keep the same people involved throughout the project to maintain momentum and so that you achieve consistency in terms of vision and approach.
Cheers
R