rob_h

No wonder you're confused. You're getting a lot of the terminology wrong. 
 
If you had CR1 why did you need adjustment of status? 
 
The three year marriage anniversary is of no importance. It's the two year anniversary which determines conditional green card or green card without conditions. 
 
I also believe your card has an incorrect date as you entered with a CR1 on 5/5/2016 and that should be the date on your card. Compare to your endorsement stamp on your visa which is in your passport. It should be the same date. 
 
You can always use this however you're already eligible to apply
 
https://www.uscis.gov/forms/uscis-early-filing-calculator
 
I had a wrong date on my Green card and it caused problems with my N400 application. I would suggest to double check. Hopefully it will be immaterial since the dates are close. 

The easy way is look at that date and add three years if you stayed married to the same US citizen. You have been eligible since May. Apply any time you like.
 
if you did not stay married to the same USC, then you would be eligible five years from that "resident since" date.
 
 

This info is all in @rob_h‘s timeline, and the filing eligibility date was 02/08/2019 (90 days before 05/09/2019)

Awesome. My comments are in the quote below:
Good luck!

Thanks CJKylie, much appreciated.
 
Thanks, re the 'Category' it was a CR1 as we were married <2 years, hence the 'I-751' was filed on 8th February 2018 to transition to a permanent resident. The date entering the US should be 9th May 2016.
 
Thanks Wuozopo, much appreciated.
 
 

Rob-- you are posting in the wrong forum so are getting incorrect advice. You are a DCF filer and the NVC, RFE, cover sheet, a $120 fee, and mailing anything are not part of your process. There is a DCF forum and a UK forum.Your affidavits of support go nowhere but to London, in your hand, on the day of the interview. I would suggest starting over with a brand new I-864 from the inlaws mailed directly to you.
Celeste must do an I-864 as the primary sponsor, even if she puts zero current income. She must provide her latest tax return or a signed statement why she was not required to file. Being a full time student and earning zero income is a valid reason. Read the actual instructions for the I-864 under the section about tax returns for more details. Celeste will also show how she never gave up her domicile in the US or her intent to reestablish domicile in the US. That's because anyone providing an I-864 is required to be domiciled in the US.
The father needs to complete an I-864, provide the required tax documentation, other proof of income, and prove he is a US citizen with a photocopy of his birth certificate or US passport.
To get an interview in London all you need to do (in any order)
1) Submit the DS-260 online
2) Have a medical exam.
3) Complete the online submission Readiness for Interview form when you have all the documents and I-864s in hand that will go to the interview with you. It says basically "I'm ready now."
All three of those things must be received at the embassy before you will be given an interview date by London.
Your steps are more similar to a K1 London process than the CR1s who filed with USCIS at the Chicago Lockbox. Your difference is your form numbers DS-260 and I-864. ( K1s do DS-160 and I-134)

hi
your wife will get an rfe
in the first place, did she already pay the $120 and get the cover sheet for the affidavit of support, they weren't supposed to send it to the NVC
because of regardless of what she makes, your wife as the sponsor MUST submit the i864 as the primary sponsor,
she will fill out the i864 as the sponsor, her father will fill out an i864 as the joint sponsor and his wife the i864A as his household member
all the forms and financial documents as evidence are sent together with the cover sheet
all USC are required to file taxes, even if they live abroad, unless the make the minimum, in that case she has to attach a letter to her i864 stating why she wasn't required to file taxes
you need to gather her parent's i864 because you need to have everything together