Why is it claimed that Betz's limit is a law?

[russ_watters]

For Darrieus type turbines, the stresses vary during each turn. You do have a notion of time. It is possible to transform constraints into additional energy recovery.

Time varying stress still isn't energy.

I agree with @Arjan82 ; at best, you are trying to use the same energy twice, and glossing-over it.

Since evidently this is your own unpublished "personal theory", it is not appropriate for discussion on PF. If you get it published in a reputable journal we can discuss it further, but I don't think publication is likely since it appears to suggest a violation of conservation of energy.