JeinKa

In order to get an immigrant visa for a step child, your wife would need to get written permission for the child to immigrate permanently to the US....OR....have a court order awarding her full custody.  Time to talk to the child's father.
 

You can't waive the parental rights of a court ordered judgement. Immigration is going to see it in black and white when you submit the divorce decree. They will not override a decision like that. So if you try to file for the child without proper permission, it would be denied.
 
The only options available, like @pushbrk stated, is the father giving permission for the child to immigrate or have a court order. And to be honest, the latter would likely be more difficult of the two options.

Note that a petition can be filed and approved without the needed custody/permission documentation, but the visa will not be issued without it.

Yes, your package will most likely be rejected and returned to you.  By the way, the instructions specifically say -- "On the “Pay to the Order of” line, write “U.S. Department of Homeland Security” (not “USDHS” or “DHS”)."
 

No plans needed, would be hard to plan for a moving date. 
I was asked if we had plans at my interview. I just simply stated we would do the JOP whenever I can get one booked. 

You are not required to have wedding plans in the I-129F filing.  Each you you are required to provide a signed and dated statement that says you intend to marry the other person in the USA before the K-1 authorized stay of 90 days expires.  
 
It is impossible to set a wedding date with the fiancé(e) petition / visa process.  
 
At the K-1 interview some (expletive deleted) COs ask about wedding plans. This is a stupid question on their part and the best answer is that with uncertainty around Covid and its impact on county marriage license bureaus,  the couple will find the most expedient way to get married and get a marriage certificate before the 90 day authorized stay is completed. 
 
You don’t  need look elsewhere for guidance on this process. Just follow https://www.visajourney.com/guides/

What country?
 
The basic requirements are quite simple.
 
Per USCIS:  https://www.uscis.gov/i-129f
 
- Evidence of your U.S. citizenship:
 
- Evidence you or your fiancé legally terminated any previous marriages (if applicable):
 
- One color passport-style photograph of yourself and one for your fiancé taken within 30 days of you filing this petition; 
 
- Evidence of legal name change (if applicable);
 
- Evidence you and your fiancé(e) intend to marry within 90 days of their admission into the United States as a K-1 nonimmigrant
 
- Evidence you met your fiancé(e) in-person within two years of you filing your Form I-129F. 
 
- If you met your fiance through a marriage broker, then Evidence that the broker followed IMBRA guidelines.
 
 
For certain places like Morocco and other MENA countries, it is recommended.to "front load" the petition with evidence of genuine relationship (meeting families, daily chats, frequent visits, etc).  But this is to have the evidence in the petition when it is sent to the embassy - USCIS does not need this to approve the petition.

No wedding plans are needed in the petition file, as others have said.
 
Some consular officers (even those who are not "... (expletive deleted) COs)...") may ask about wedding plans during the visa interview as a way to find out more about the bona-fide relationship of the couple.  Even though there can't be an exact date set or invitations printed, it would be very unusual for a couple not to have discussed some general plans for getting married -- for example, we plan to get married at the courthouse and have a more formal ceremony after we have time to arrange it; or, we'll get married at his/her family home as soon as we can get a license; or, his clergy has said he will marry us  in the church whenever we are ready; or, we've talked a lot about getting married and how we would like to do it but haven't made any definite plans until we get the visa.  An engaged couple who have not had any discussions at all about getting married would be unusual.