Ok so here are some answers some from me and many based on Rav Yosef Deutsch's Let me join your nation (Feldheim)...
- The reason Rus went at night specifically was because it was a given that Boaz would be there and it was good time to "catch him". This was because:
- Rashi Rus 3:2 - theft and robbery was rife and it is normal for the owner to sleep in his granary to protect it
- Chullin 91b - Boaz was a known talmud chacham and thus Boaz would not return home from the threshing floor, as a Torah scholar should not go out alone at night.
The reason why she presented herself in the way she did, uncovering his feet and lying there was because
- Malbim: The spirit of Machlon would not rest until she performed yibum with one of his relatives. If he didn't want to do the levirate marriage the candidate has to do chalitzah which is performed through removing the shoe of the said relative. Thus uncovering Boaz's feet was a remez (a hint) that he must either enter in marriage or do chalitzah instead.
- Alshich - Since she made a point to lie down at his feet and not near his head it would rather signal her humility. Additionally the fact that she was clothed and tznius would show that she had pure intentions.
The expression of "spreading the cloak" carries the reference to marriage as Rashi says it was "לְשׁוֹן נִשּׂוּאִין" - an expression of marriage. So once she explained her reason that she acted on the advice of her mother-in-law, and that Boaz accept his duty (to either do yibum or chalitza), he should make his decision accordingly.
- The reason Boaz didn't send her home because:
- So why didn't Boaz leave? Well firstly, if it was dangerous for Rus, it also would have been dangerous for Boaz as well.
Indeed, he did wrestle with his Yetzer hara see Sifrei, Bamidbar 88:
Similarly, (Ruth 2:13) "As the L-rd lives, lie (here) until the morning." Because the yetzer hara (the evil inclination) sat and aggrieved him (Boaz) the whole night, saying: You are single and need a wife, and she (Ruth) is single and needs a husband, and you know that a woman is acquired (as a wife) by intercourse — Arise and live with her and let her be your wife — he swore to his yetzer hara "As the L-rd lives," I will not touch her. And to the woman he said: "Lie (here) until the morning."
But he was worried significantly about what people would think and so likewise stayed. Rav Deutsch describes the situation very clearly:
They sat up together in companionable silence for the rest of the night; Ruth absorbed in her thoughts, Boaz preoccupied with the pitfalls of the situation (Igeres Shemuel on Ruth 3:14).
Here he was: an eminent judge, the acknowledge leader of the Jewish nation, for all intents and purposes alone on a darkened threshing floor with a woman to whom he was not wed. What if people find out? What would they think? What would they say? Would his reputation be ruined? Would Ruth's? Would the honour of Heaven be desecrated? (Ruth Rabbah 7:1 with Yefeh Anaf, Tiferes Tzion)
It is worth noting (at least according to Rabbi Deutsch based on the Shoresh Yishai on Rus 3:13) that Boaz offered her the place to sleep whilst he offered to lie down elsewhere. However she averred and requested that she just sit down as she had plenty to think about (based on Igeres Shemuel on Ruth 3:11 and Eshkol HaKofer on Ruth 3:14).
- This has already been answered well in the comments by @Alex based on the Gemara at the bottom of Sanhedrin 21a - 21b:
Yichud between two unmarried people hadn't yet been prohibited; this was done after the incident with Amnon and Tamar, several generations later
- So nothing is in Tanach without reason and I would like to suggest an idea that perhaps makes this episode warrant mention.
As I write this during the period of Shovavim Tat, a period where we take on to be more scrupulous in inyanei tahorah (matters of purity) and shemiras einayim (protecting ones eyes) this incident seems particularly apropos.
The Tikkunei Zohar 31:75b writes expressly about Boaz the following:
דְאִיהוּ בּוֹעַז בּ''וֹ עַ''ז תַּקִּיף עַל יִצְרֵיהּ
That Boaz (is an amalgam of the words בו עז) "in him there was strength" - he mastered his evil inclination.
Indeed, the Gemara in Sanhedrin 19b writes:
Rabbi Yoḥanan says: Joseph’s power is the humility of Boaz, as Joseph is praised for showing strength with regard to an accomplishment that was insignificant for Boaz (see Genesis, chapter 39). Likewise, Boaz’s power is the humility of Palti, son of Laish, as Palti’s capacity for restraint was greater still. Joseph’s power is the humility of Boaz, as it is written about Boaz: “And it came to pass at midnight that the man was startled and turned himself, and behold, a woman lay at his feet” (Ruth 3:8). What is the meaning of “and turned himself [vayyilafet]”? Rav says: The meaning is that his flesh became like the heads of turnips [lefatot], his sexual organ hardening out of arousal, but even though Ruth was not married he refrained from engaging in intercourse with her; while Joseph had to exert more effort, despite the fact that Potiphar’s wife was married. (Sefaria translation & notation)
So we see this episode with Boaz and Rus was one of great temptation, one that exceeded that of Yosef and Potifera and yet Boaz passed the test. Thus it warrants mention as it is another important lesson in overcoming the lure of the Yetzer Hara.