No this is not considered a public benefit.

Your legal status should not make a difference for purchasing CD.

This appears to be a no based on information provided.

You can list the primary address of your employer.

Yes you should have 87 days for response to RFE.

Best to provide the most recently filed tax return.

You will need to establish requisite income for affidavit of support purposes.

No it is best for USCIS to contact you and request the evidence.

Yes you may do so through the AR11 form.

Complete the COA for the 3 months and then upon return.