Dependent on H4 and already has H4 EAD. Ready to file I765 along I485 which category to be used on I765?

C 9 is almost always used when applying for AOS.

Am I eligible for adjustment of status?

You could indeed be, based on the advice Mr. Behar gave you.

EB-2 to EB-3 downgrade and concurrent i-485 filling?

I agree. Based on this and incessant, repetitive questions on the matter it becomes clear that CIS will raise many 'red flags" on both you and the...

Nothing of the sort to worry about when applying for AOS as the spouse or a USC.

My wife and I are on federal Disability. Our niece is a Canada citizen we need her to be live in caregiver and American citizen?

Not possible - and that for a multitude of reasons.

Employment-based applicants do not need to file I-864.

Had USCIS approved the case, it would have been clearly so reflected on the case status site. "Decision" normally connected to Denial.

Will i539 gets deny when its applied after i129 approval?

You don't need to fear a "denial" based on the facts recounted here.

When to add Spouse in my GC process

You can only include (mention/list) your spouse at the I-140 stage; not before.

How does the process work for EB2 to EB3 downgrade for immigration related matter?

The employer is the petitioner. As such only the employer can act on this one.