So, apparently the top came out almost symmetric.
Of course the brace became loose again. But i admit a was not unhappy at all about that; the reglue occured accidentally and the fit was everything but good.
Through my loupes i saw that it was indeed the laminate which tore off over more than half the length of the brace, except of abount the first inch. The surface of the glue layer could be clearly seen through the gap, almost everything seemed to have remained mostly intact on the inner laminate layer. And hence unreachable.
I decided to reglue the brace with fish glue, thinned just as much that it could be injected through a 1.2 mm needle. Please note the four holes i drilled into the gap in order to be able to inject the glue even there.

Admittedly i do not trust the stability of this joint - although i expect fish glue to perfom better on the rests of the older glues that for example CA. So i'll add some kind of lining, maybe cut from the rests of the bars of the thinline.