1. If you are extracting only the standard cell, you should probably do flat extraction. If you are doing a design with standard cells, gate-level is a better fit -- but you will need models to provide cell values for STA. (Gate-level extraction is just the portion of the design outside the cells listed in the xcell list; for many designs, that's interconnect and some glue logic.)
2. Whether it can be used for sign-off depends on the foundry's requirements. I know of no technical reason it cannot be used, but my experience was with larger (>90 nm) process nodes. I believe Mentor acquired Pextra (the source for the current Calibre Xact) because it was developed specifically for the extra requirements of small nodes. Check with the group who will be doing the manufacturing what they require for sign-off qualification.
Best regards-
Sam.