Sorry to be so late weighing in. I've been on vacation and out of internet range.
I've tried several times to adapt a favorite film to a play without success. There are just too many structural differences to make an adaptation. First, movies are about pictures, plays are about words. Movies have many very short scenes, plays have a few longer scenes. Movies use many characters in short walk on roles, plays have fewer characters and today it is very hard to cast people for a very small role. (Think of the police at the end of Wait Until Dark.)
If you are looking at a realistic set, you are limited to one or, at most, two locations. Small theaters don't have the space or money for multiple sets. If you want suggested sets, there is more flexibility. Remember that even in suggested sets, there has to be something that signifies the specific scene so we will recognize it when we return. It can be something as simple and a tapestry on the wall, a chair, a painting or just a box. But remember that it takes time to make the scene change - even more so for costume changes. Here is a rule of thumb: You will lose the audience involvement in a play for three to five minutes after a scene change. Many short scenes mean a lot of time when the audience is getting back into the play.
The days of large casts are gone. Here are the cast sizes for our last two years: 8, 6, 4, 6, 2,2,11,2,2,5,5. The 8 and 11 are our winter shows when we try to use newer, less experienced actors. Our stage is 26' wide by 16' deep. It is extremely difficult to get 11 people on stage at one time and not have the actors stacked like cordwood.
Flashbacks are not difficult unless you need a major set. In one case, I had a wall and platform on wheels that rotated with another set on the other side. In another case, I had an empty area on stage and was able to move a very small table and two chairs into that area and lit it in such a way that the rest of the set was not lit. One play that is on my someday list has two narrators who comment on what is happening on stage. I would build a small thrust off to one side of the stage and sit the two characters there.
My advice is to not adapt the screenplay but write a new play from scratch. Same characters, same premise, same issues but written for the stage not the screen. It sounds like your screenplay is about ideas, words and relationships. It probably would make a better play.
Reading between the lines, I get the idea that people have told you that your screenplay is not a great screenplay. My gut feel is that, if you start from a less than great screenplay, you will end up with a less than great stage play. Start from scratch.
Just my two cents worth.
Doug