I don't recall the source for this but I remember learning that when she saw that she wasn't being given in marriage to Shelah there was a tacit understanding that there was no intention on Yehuda's part for her to ever marry him.
if Shelah has reached maturity why doesn't she go to him directly?
Because he is still under the dominion of his father Yehuda
Did she have a 'plan b' if Yehuda had not opted to sleep with her when he thought she was a random prostitute?
I am not aware of any commentator who says she did have a plan b, why would she?
Why not reveal herself to Yehuda right away and explain her actions?
How would revealing herself serve any purpose? She doesn't even know if she is pregnant, which is ostensibly the point of sleeping with Yehuda. Perhaps you mean to ask why she doesn't come forward when she misses her first period.
It would stand to reason that she doesn't believe Yehuda wants the same thing she wants, ie to further Er's family line at any cost. As such it would seem that she may have not even been living among Yehuda's family anymore as it was others (Bereshis 38:24) who brought her pregnancy to his attention. Perhaps she planned to raise the child alone.