kooshball

Your immigrant visa, once endorsed by CBP, serves as temporary proof of LPR status for up to one year. 
 
You can travel and work using it without issue until the year is up. 

USCIS guidance https://www.uscis.gov/i-9-central/temporary-i-551-stamps-and-mrivs

Exactly. I returned couple months later - with just endorsed visa that served as a temp green card. 

I did. Wasn't a problem. 

your dad will file the I-864 as the only joint sponsor.
you are the petitioner as well as the primary sponsor on your I-864, but you dont have income above 125% poverty line/us derived income and hence the need for the joint sponsor.
 
Joint sponsor could be anyone, living domiciled anywhere in the USA as long as they have the requisite income and tax transcripts to substantiate the same.
 
Along with the I-864 , he needs to send you his 1040 for the previous relevant year definitely (but if he send for previous 3 years that while not necessary, may come in use). In case he sends the 1040, then he needs to send every document he used to file the 1040 , which includes all w2s and 1099s.
OR
He can simply send you the tax transcripts for the relevant year. This is preferred and simplest.
 
NOTE: In case he files married filing jointly, and this i say from personal experience as it was brought up in my case by the interviewing officer, then even with the transcript, you will have to submit his w2s/1099s to demonstrate, that he-the sponsor signing does indeed have the necessary income individuallly and its not derived from his spouse and also a plain statement of the spouse saying the she is aware that her spouse is filing an i-864 using their common returns as proof and that she has no objection.
 
Seems complicated, but is actually quite logical and easy to put together.
Best of luck.

Your dad is the only Joint Sponsor.  
 
You are the Petitioner / Sponsor.  You are not a Joint Sponsor.