I think you are overthinking this. I am a native speaker and I usually only pronounce "she" like "sheeeeeeeeeeeeeeeeee", unless it is part of a contraction like "she'll" For example: "She'll be arriving shortly." Good luck!

I'm a native US speaker and I only know one pronunciation of "she." I tried saying sentences with the word "she" as fast as I could, and in as relaxed a way as I could, and I still pronounced it "shee." To confirm this, I checked an American dictionary, and noticed that as I expected, there are two pronunciations for "the," but only one for "she." I believe there is only one pronunciation of the word "she."

two sounds, sh and i (ee)

I agree with Ian. What do you mean by weak form? I'm not aware of any other forms of 'she'.