To answer some of your questions, the Midrash Tanhuma (ed. Buber) to Parashat Vayetseh (12) explains that this was an arrangement that was made by their respective families:
כיון שילדה רבקה עשו ויעקב, נולדו ללבן שתי בנות לאה ורחל, שלחו אגרות אלו לאלו והתנו ביניהן, כדי שיטול עשו את לאה, ויעקב רחל, והיתה לאה שואלת במעשה עשו, והיתה שומעת שמעשיו רעים, והיתה בוכה בכל שעה לומר כך עלה גורלי לרשע הזה, ומתוך כך נעשו עיניה רכות
When Rivka gave birth to Esav and Ya'akov, Lavan had two daughters; Leah and Rahel. They sent letters back and forth and arranged that Esav marry Leah and Ya'akov marry Rahel.
The Midrash goes on to say that Leah inquired about Esav, and was told he did bad things, and therefore cried.
There is no indication from this Midrash that were Leah to marry Ya'akov, that Rahel would have been coerced to marry Esav. If anything, the implication is that once the initial arrangement was broken, they would have to communicate once again to decide how to proceed, rather than there being some mechanism that would've kicked in.