Your description would work. Many marriage agencies arrange socials where ALL the woemn meet and have their phtographs taken with ALL the men present. If something develops later the agncy will produce the needed photo (for a fee of course) Letters of intent can be signed and mailed later. The G-325 does not have to be signed, the petitioner can fill it out (I can send you the internal USCIS memo about that if you didn't already know it) You do not need a copy of the beneficiary's birth certificate for the petition. Only the petitioner needs that and ONLY if they are using it to prove citizenship, A copy of your passport works just as well.
Read? The OP asked if he needed additional "proof of relationship" and if he could send it. I answered correctly that he did not need ANY proof of relationship for the petition. The OP has a fiancee from Ukraine. Proof of relationship has NO affect (NONE!) on the approval of a petition. It MAY have an affect in some difficult consulates. The OP is not going through one of those. My answer to the OPs question was correct and is not "bad advice". Pictures of the family of your fiancee would have NO affect on the petition process or on the visa process in Ukraine. Kiev couldn't possibly care less if the beneficiary even has a family.
In the case of the OP, it is not possible to even imagine a need for any proof of relationship submitted with the petition. Save it for the interview.
Your advice confuses the steps in the procedure and I hope it does not cause the OP any unnecessary worry.
