R&N2019

Same thing happened to us in Detroit - interview was Nov 13.   We think case officer was in training because someone observed her and gave her direction at times.  All went well and at the end she said she needed to review case.   I mentioned I hoped to have my green card to go back to Canada at Christmas and she said that shouldn’t be a problem, but reminded us she had 120 days to review file.   On Nov 22 our case finally changed to approved.  

IOs (unless your IO is a supervisor) will never tell you that you are approved at the interview. If they are not a supervisor, they wont give you a I551 stamp either if you ask. This is the normal routine.

You can. No, it won't affect anything. They probably will not, if the approval of the green card is imminent. In fact, I have seen cases where they will deny the EAD while approving the green card shortly after. And I've also seen cases where they would approve the EAD and GC approval follows by several weeks. So there is really no logic to any of this. 

In this case, I would submit BOTH the complete tax return AND a tax return transcript.  Note that since your sponsor is both employed AND self employed, his "current income" will be what appears on line 6 of the 2018 tax return and past income will come from line 22 of the previous two returns.  
 
THAT, is his "income".  If it is insufficient, get a qualified joint sponsor.
 
Note:  You posted your question in the spouse visa forum.  This is NOT a visa case.  It's an adjustment of status.

Sounds like some tax forms were missing. I would start there by providing a full copy of the entire tax return.
The second bullet point is automatic upon the first one.
 
I doubt credit score would be beneficial. I think you're reading more into what is being requested than what is on the RFE.

If they don't have all the information to support the income, then they don't consider it at all and therefore the income is insufficient.
 
Generally, the tax transcripts are suggested as it's simpler / less documents to miss. That said, go by the wording on the RFE for whatever they request.
 
Assets are optional if income is sufficient. Assets need to be fully documented - ownership, net value, etc., per the I-864. In the case of something like vehicles w/o a NADA/KBB value, an appraisal would generally be the closest way to determine their value.
 
That said, I'll reiterate that if income is sufficient, then assets are optional. I would focus on providing what was requested, which presumably was documentation. Adding more complexity (a joint sponsor, assets, etc.) means more documentation is needed.
 
Generally, expect only a single RFE for a particular topic. I have seen people get more than one with the I-864, but anticipate that you only get 1 shot.