(i) I have no specific figures, but the Targa is stiffer than the cab. To be 100% honest, it might be better, as the following factoids show:
The -20mm option will not be available on the Targa either because it is too hard on the frame (by Porsche standards. Other manufacturers might not care

).
Also, the roof has to be regularly re-adjusted to avoid rattles. The glass panels and the steel beams do not absorb energy at the same rate. As a consequence, the mechanism gradually loosens - hence the extra maintenance.
(ii) True. True parallelism would be impossible given the shape of the roof. Related downsides: when the roof is open, rear vision is bad, and rear passengers get a *close* hair cut.
(iii) True. On the plus side, Targas are very exclusive, and immediately stand out in a sea of regular coupés.
The concept has its drawbacks, but the glass roof is definitely worth it IMHO.