1)You can both sign the letter. Address them as father and mother.
2) You wife can sign herself.
3) You should fill in the 1-134 form. Somewhere in this form it asks for annual compensation etc. It is better you fill it.
4) No more. But they should bring the marriage cert for you and your wife.
We are in the same situation. The focus is to tell the interviewer that your in laws will return to China after their visit. If this is their first interview, they will very likely get their visas. Good luck