Hey!
My Fiance is going through the Frankfurt consulate as well.
1. He filled out the DS-160 but waited to submit it until he got the letter from the consulate with instructions. Once you start filling out the DS-160, it stays saved for 30 days. So as long as you keep the application ID it gives you at the beginning, you can log back in a finish it when the time comes. Our case was changed to ready status on April 11th, and he got his instructions letter on April 25th. It can take anywhere from 10 days - 2 months to get this letter from them. We read some places that they got the letter after 7 days.
2. Once you get the letter from the consulate, it tells you to email the confirmation to them. But also to bring it with you to the interview. Emailing it to them just informs them that you're ready to schedule the interview. It's best to just bring the confirmation with you anyway. Better to be over prepared than under prepared.
3. In the instructions letter that my Fiance got, it told him to bring all of the documents to his interview and NOT to mail them in. I doubt this is one of those "case by case" things where some people mail it in and some don't. So you should have time still to finish collect documents since you will be bringing them to the interview.
4. I assume you'll be scheduling your medical with the doctors in Frankfurt? If so the soonest appointments they had (last time we checked) was late June. While the soonest interview appointments at the consulate were as early as May 8th. In the instructions you get AFTER emailing the DS-160 to the consulate, it does say you can do the medical after the interview. We personally scheduled it so the medical is first. I have read from multiple different people (when talking about the Frankfurt consulate) that it can take longer by doing the medical second, and they have had issues with the doctors getting the information to the consulate quickly or without issue.
You can schedule the medical exam without having proof of an interview scheduled. but you DO have to bring a print out of the interview appointment confirmation to the medical exam. So either way, you do have either have the interview scheduled or have already done the interview to do the medical exam.