How did Le'ah know when Yakov returned home that evening, to guide him to her tent?
The Gemara in Nidah (31a) refers to this episode to explain the Pasuk (Bereishis 49:14) "Yisachar Chamor Garem" (Yisachar is a bony donkey), which can also be translated as "A donkey caused Yisachar (to be born). See also Pirush Yonasan.
Why did Le'ah mention the fact that 'she had hired Yakov ... '? Why was it not lacking in Tzeniyus to talk like that?
Seforno: Because it was in order to reassure Yakov that she was not encroaching on her sister's rights, seeing as Rachel herself had agreed to allot that night to her. 1 We see from this statement, indeed from the entire episode, that the Avos and the Imahos treated intimacy like Adam and Chavah before the sin - to have children in the service of Hashem and not for personal pleasure.
The Gemara in Eruvin, 100b actually learns from here that a woman who takes the initiative and hints to her husband that she wishes to be intimate (See Torah Temimah, note 4) merits to have children who are 'Nevonim' (wise and understanding), like Yisachar (who was conceived on that night, and about whom the Pasuk writes in Divrei Hayamim 1, 12: "u'mi'Benei Yisachar Yod'ei Binah le'Itim". See Torah Temimah.
Why does the Torah omit the first 'Hei' in "ba'Laylah 'Hu' "?
Seforno: "Hu" refers to Yakov, to inform us that, when he saw how keen Le'ah was to be with him, and positive motivations, he actually accepted her wholeheartedly. 3
Yerushalmi Sotah, 3:4: "Hu" (one of the Names of Hashem) implies that only Hashem knew that Le'ah sole intention was to produce tribes.
Tosfos ha'Shalem (6): Yakov would always go to the tent in which he saw the Shechinah.
Tosfos ha'Shalem (2): This shows that Leah was not considered 'Chatzufah' (a wife who verbally requests to have Bi'ah, one of the nine Midos of despised relations).
A large step forward from the 'hatred' that he previously bore her.
Why is there a double expression "Sachor Secharticha"?
Tosfos ha'Shalem (4): This hints that Yisachar, who was conceived that night, will engage in Torah and receive reward in this world and in the world to come.
Rashbam: Le'ah 'gained' Yakov on two scores - on account of the Duda'im that she gave Rachel and on account of the fact that she gave him her maidservant.
רש"י: שכר שכרתיך - נתתי לרחל שכרה: למה לא פירש כפשוטו?
גור אריה: לאה לא נתנה ליעקב דבר, אלא 'נתתי לרחל שכרה'.
למה נולד יששכר בזכות הדודאים?
מהר"ל, חידושי אגדות (סנהדרין דף צט. עמ' רכט, ד"ה שאין): אשה שתובעת בעלה לתשמיש זוכה לבנים תלמידי חכמים כיששכר, ומתוך שהשתוקקה לאה להזדווג ליעקב- נתנה דודאים תמורת הדודים [-הזיווג] של בעלה, ולכן זכתה בבן כיששכר. 1
במהר"ל (גבורות ה' פמ"ג, עמ' קסב, ד"ה המאמר) מבואר שלאשה יש מידה חומרית ולאיש מעלת הצורה [חומר= האפשרות הנמצאת בכח, צורה= ההוצאה מן הכח אל הפועל], וע"י שהיא משתוקקת לבעלה היא דביקה בצוה ומשלימה את עצמה ולכן כתוב גמ' בנדרים (דף כ:) שיששכר שהיה בעל תורה נולד בזכות תשוקה זו.