1. How can we model offsets(connecting two corner nodes) in plates, I used an element with the same thickness but w 10x the E, Is this reasonable enough or what are the other ways?
2. Is there a way to rotate the local axis (x and y axis) of elements?
i) You can not model offsets in plates. Offsets are meant for member.
ii) Yes, there is a way to rotate local axis of elements. You can refer to Help or Technical reference manual.
iii) Mxy stands for torsion. Staad does not design plates for torsion.
Regarding item (3) in Mr.Sureshsharma's reply,
For slab design, in the RC Designer mode or in the Advanced slab design mode, there is an option to use Wood and Armer equations to convert Mxy to an Mx and MY and then use those values in the design.