Some disability questions

This isn't for me, this is for a family member, and I'd like to help give them some decent advice. Admittedly, I know absolutely nothing about disability. (neither do they)

They are at the point of considering disability for mental health. I'm pretty sure they would qualify and are deserving, but they also want to fight thru it....

1) What is the payment for SS? The online calculator said about $2500 a month, but I read the max was $914 for mental health? I know it is income based, but is there a max limit based on the issue?

2) Can you draw employer based long term disability and SS? Is employer disability for a lifetime? Or a certain period?

3) Can you still work some while on employer or SS disability?

4) I think they could go on their spouse's insurance, not sure if that matters.

5) Anything else they might need to know?

These might be pretty basic questions, but reading online is gives conflicting and confusing answers.