Why is the 'Kuf' in the word "Katzti" small?
Ba'al ha'Turim: Because she foresaw that Esav's descendents (Edom) were going to destroy the Beis-Ha'Mikdash, which was 100 Amos tall. 1
Ha'amek Davar, Oznayim la'Torah: Because this was not the real reason 2 that she wanted Yitzchak's consent to send Yaakov away; it was only an excuse. Oznayim la'Torah - it seems that Yaakov would not leave Eretz Kena'an without his father's consent, so Rivkah fabricated an excuse for Yitzchak to order him to go to Padan Aram.
By the same token, David, in Tehilim (84:4) wrote the word "Ken" in the Pasuk "u'Deror Ken Lah") with a large 'Kuf' - because he was praying that the Beis-Ha'Mikdash (which was 100 Amos tall) should not be destroyed.
Ha'amek Davar - she was already used to Resha'im; the evil of Esav's wives did not pain her. Esav's wives vexed her because they recognized that she does not love Esav. This would not apply to Yaakov's wives, even if they were Benos Ches! Also see Oznayim la'Torah.
Why did Rivkah not tell Yitzchak the real reason behind the need to send Yaakov to Charan?
Rashbam: She acted wisely in giving the reason that she did without telling him that it was on account of Esav's hatred. 1
Ohr ha'Chayim: It was due to the Isur of Rechilus. 2 She needed to tell Yaakov to save his life.
Malbim: She did not want to pain Yitzchak.
Refer to 25:28:4:1 and the note there. Presumably based on the fact that Esav's plans were revealed to her and not to Yitzchak, she realized that HaSh-m wanted Esav's enmity to be withheld from him until the day he died, despite the fact that he now knew about his evil (Refer to 27:33:1:1 and 27:33:1:4). Moreover, Esav knew why Yitzchak sent Yaakov to Charan and reacted accordingly. Who knows how he would have reacted had Yitzchak sent Yaakov away to save him from Esav's hands! (EC)
This is astounding. Even if Esav were considered "Amecha," he is a Rodef! If she will not tell Yitzchak, he will be worried why Yaakov delays returning, or suspect that he neglects Kibud Av v'Em without need!
Surely she would not fear to tell him information he does not know. However, if he would say 'I know Esav - he would not really kill him,' she would not be able to disagree! (PF)
Why did Rivkah use the double expression "mi'Benos Cheis, mi'Benos ha'Aretz"?
Oznayim la'Torah: Because there were two points against the Benos Cheis 1 : (1) They were all idolaters who had been raised from the cradle to sacrifice to idols, and (2) They lived among their relatives, who would encourage them to keep up their idolatrous practices, rendering it impossible to train them to serve HaSh-m. 2
Malbim: Even if she would be from Benos ha'Aretz not from Benos Ches, if she would be like these [wives of Esav] ....