Ok...I am no expert and by no means do I know more about pe than some of the people on this board...but I will try to answer your questions.
I think personally and from some experience, that stretching while flaccid is better. You can stretch at more intense angles while flaccid, and can stretch the ligs and tunica better while flaccid. There are more exercises it seems for flaccid state stretches. You also can hang while flaccid...it just seems that being flaccid and stretching is more versatile and worth while...I do not know much about erect stretching so I cannot go there.
As for the kegel. I don't think it matters if you are erect while doing this or not...remember what you are doing...you are exercising the PC muscle, not really the penis. If you do kegel while erect, you will notice a rush of blood engorging the glands....this is useful in certain exercises when needed. However, kegeling can be done erect or flaccid...I think its more for preference.
As for masturbation having a negative effect on pe....when you say Penis Enlargement that is very broad....do you mean jelqing, stretching, hanging, clamping, ect? I think that if you do a routine, nice and intense and masturbate at the end and ejaculate (I am assuming you mean ejaculating by saying masturbation) you will be ok. I think though after you do this, you do some quick jelqs and stretches to keep the penis from constricting from the orgasm....When you have an orgasm, hundreds of movements in the penis (muscles or nerves?) shake in a vibrating movement causing intense satisfaction (orgasm). I can conclude that this would cause the penis to constrict afterwords from fatigue...This being said, you wouldn't want the penis in this state, but always in a stretched and enlarged state....