From what I understand from The Midrash Says, Rachel knew she was in-line to marry Yaakov, and Leah was supposed to marry Esav. And that's why Leah cried a lot.
When Rachel allowed Leah to marry Yaakov, what did she think was going to be her fate; Did she think she'll have to marry Esav, or did she think she'll eventually marry Yaakov?
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.
Eisov was already (happily) married for 44 years! He married when he was 40, and he had no plans to have Leah at all. Yaakov married at 84 (as we know).
After Yaakov stole Eisov's blessings, it was clear that he now has to fulfill Eisov's part in building the Jewish nations, therefore marrying both Rachel and Leah and fathering the 12 tribes (originally Eisov had an option to be the father of the 12 tribes, hence his status as Israel Mumar).
This intention of Yaakov of marrying both sisters is clear in numerous Midrashim and interpreters (see Midrash Rabbah, Yalkut Shimoni and more). The only importance to Yaakov was the sequence, not the question of whom.
Rachel also knew this intention as they surely spoke for the 7 years! However, she was afraid that because of his love for her he will forget of fulfilling the Eisov's part and will not marry Leah at all as the Posuk says "בְּאַהֲבָתָהּ תִּשְׁגֶּה תָמִיד" (Mishle).
Therefore Rachel was sure she will marry Yaakov anyway even if she plays with Lavan's evil intention to undermine the sequence.
FYI, it is completely unclear how Yaakov married Leah, and if he married Leah at all, as he paid Lavan twice for Rachel, and slept with Rachel with that intention, but his sleeping with Leah without intention to marry does not count at all (according to existing Halakhah at least).
