That is a very important observation that makes a lot of sence. As the batten is flexed the steel in the inside will move relative to the one in the outside creating extra force against the end of the the protective covering. Fortunately this potential problem can be completely prevented with proper design. 1) make the inner part of the batten a little shorter than the one outside and don't use long battens: if used perpendicular to the leach they will be smaller than the vertical or horizontal ones for the same leach. 2) making sure the ends are rounded and covered with plastic to prevent the shafe. 3) never leave them partially furled as they will continuously be flexing and likely will fatigue. This is a true draw back as it puts a design limitation on the infinite number of reefs that the in mast furling sail can have without battens or with vertical battens.
Thank you for bringing up this key point . I will be happy to be a guiney pig with this design as I cant see myself puting rigid vertical battens on my amel.
Some times innoavation comes through those who are willing to take the risks.