"won't" and "wouldn't" in the context you present in your original question mean "refuse(s) to" and "refused to", respectively.
Thus, "She wouldn't let me do ..." is equivalent to "She refused to let me do ...". Again, in the context shown, this has very much the same meaning as "She didn't let me do ...".
I'd be hard-pressed to decide which shows the greater degree of certitude. With "wouldn't" we see that the subject ("she") showed more stubbornness, shall we say, or at least more emotional involvement, in not permitting a lot of things. "didn't" is clinically and coolly objective in comparison. In either case, I assume the speaker is equally certain about what he observed
1- Wouldn't contracted form of would not 2-Used to describe the consequence of an imagined event 3-Used with have+past participle to describe a hypothetical action or event in the past 4-used in making polite request 5-used with imagine, say, think, etc to give tentative opinions 6-Used in offer or invitation 7-Used
CJ ("would" does not form a subjunctive.) Thus, "She wouldn't let me do ..." is equivalent to "She refused to let me do ...". You confused me.. In *all class-*** the subjunctive mood means that speaker is considering an action as suppositional/possible/desirable... And would verb is one of the subjunctive verbs... That is I want to say that
(1.) She didn't let him go. (2.) She wouldn't let him go.
A: Mom, can I go to the playground?
Mom: No, you haven't done your homework or cleaned your room.
A report of this encounter would include 'would' because as Jim said, it involves/involved a measure of emotiveness/modality. Both 1 & 2 state the same certitude.