So, interesting info. In our case, the foreign fiancé has not yet received her letter from the Embassy, but has scheduled her biometrics, interview and just finished day 2 of her physical today.
I'll explain.
As you see in my signature block, the NVC emailed us that it was passing the packet on to Manila embassy back on 4/22 after holding it for almost a month. To this day, I have not received a physical letter to the same effect. That's deviation from expectation #1.
She had completed her DS-160 as soon as the NVC notified us that it had the packet from USCIS back in March. She was able to move forward with scheduling biometric and Embassy appointment on late 4/23, a day and a half after getting the notification email from NVC.
She set her biometric for May 7 and we got lucky that a sooner-than-expected slot was open May 13 for the interview. She then scheduled her Medical at St. Luke's 4/29 - 4/30, which she is finishing as I type this.
While sitting at Day 2 of her physical, I, the petitioner, received an email from the US Embassy in Manila instructing us that it has the packet and to complete the DS-160 and then schedule the appointments. Had we waited, we would have wasted precious days/weeks.
So, it's a bit of a gamble because the DS-160 is only valid for 30 days, but I strongly encourage you NOT to sit and wait for the embassy letter/email. We have yet to get any printed letters, petitioner or foreign fiancé, and the emails have been a bit later than past descriptions would lead you to predict.
Disclaimer: your mileage may vary, don't blame me if things doing go the same for you. I'm just relaying the particulars for OUR situation. She's on track to be her by our planned and purchased travel date of June 4.