Happyt2

I just received my citizenship and I am applying for my passport for myself and Child. he is an LPR in my full legal custody. He has only been in the US for two years and has lived with me the whole time , from what I have read I should include the Childs school and medical , our tax, MY auto registration , deeds  and other documents to prove that we have resided together at the same address for the complete two years ( we moved one ) is this correct ?   ( Mike E any suggestions ? ) 
 
Evidence the child is residing in the United States in the legal and physical custody of the U.S. citizen parent.  
 
FAQ: Child Citizenship Act of 2000
You may provide any documents which show you reside in the United States such as school or day care records, utility bills, employment records, automobile registrations, deeds or property rental leases, medical records, or passport stamps. 
 
Only children residing in the United States are eligible to automatically acquire U.S. citizenship under Section 320 of the Immigration and Nationality Act.
 
Your residence is the primary place in which you live. Entering the United States or temporarily visiting the country – even if on an immigrant visa – usually does not meet the requirement to reside in the United States. As such, legal permanent residence cards are not evidence of residing in the United States.
 
 

Earlier, later .. reschedule to what fits .. once your case file is at the embassy

No fees to the embassy.  Everything has been paid to NVC.    Just have to be patient for now

Pretty sure, yes.  I suppose the person can digitally sign the actual form, and then upload the form after printing, but the OP's question was about avoiding printing.  These forms are not completed ONLINE like the DS260 is.  DS260 is digitally signed BECAUSE it is an online form.  The I-864 etc. is a downloadable or printable PDF, not an online submission.

Using c/o you at us mailing address should be fine given he’s a minor

I don't think you can delete and upload a new one but you can take an updated set of financials to the interview.
 

I think you will do the forms on paper, print, sign and upload a scan.  

Yes, take the original signed forms to the interview.  I-864 and I-864a.
 
Certified copies of civil documents to the interview.  No passport copy or original except the one for the visa applicant is required at this stage.  A certified copy is still a copy, not an "original".

Beneficiary/Applicant is Stepchild of Petitioner???
 
One current pay stub is better than a letter from an employer.  No need for six months.  Look at it and you'll see it shows Year to Date income anyway.
You'll need information from three tax returns.  Sending in more than the latest transcript is optional. 
If you are filing taxes jointly, no need for marriage certificate or driver licenses.
Yes, best practice is to take original signed FORMS to the interview.  "Documents" will be copies anyway.

Petitioner I-864.  Joint filing spouse I-864a.  This is standard and best practice.  Just do it.