ANONLAURAY

Your I-864 getting flagged mostly depends on whether your CURRENT INCOME is above the minimum requirement.  2020 taxes are not required until April 15th.  Up to you if you want to delay submitting the I-864 because of your past income.  Based on your other posts, you are pursuing F4 visas for your family.  That visa category is very backlogged, so it won't really make much difference in their waiting time whether you submit the I-864 now or in February.
 

You need to figure out if your niece/nephew will age out or not under CSPA.  

Google CSPA calculator.

You need to know 1) the Priority Date, 2) the I-130 approval date, 3) the date the Priority Date became current, and 4) your niece's birthdate.  

 

This is a vague step, it had me scratching my head too because there were no instructions as to what they really meant. After seeing other VJ posts I uploaded a single passport-style photo (like you said, uploading 2 is redundant when filing electronically). It was accepted with no problem.

Same answer as in your other thread -- print out the form and hand-write your answer using black ink.  It's ok for some answers to be typed up and others to be handwritten.
 

Pay close attention to the size of the file. If it's too big (more than 2MB), CEAC will not allow you to upload it. JPG and JPEG files tend to be large, but are accepted by NVC along with PDFs. If it is too large, upload a PDF version.

Either you do need to use black ink.