What made Yaakov believe that his father might feel him (and discover that he was not Esav)?
Ramban: He was worried that he might bring him close to kiss him, or to stroke his face (in the way that a father does).
He was seeking an excuse not to trick his father (refer to 27:12:151:1). (PF)
Why was Yaakov only concerned lest he feel him, but not lest he recognize his voice - an obvious giveaway?
Ramban #1: Perhaps their voices were very similar, as is often the case with siblings. 1
Ramban #2: Perhaps Yaakov was able to change his voice to resemble Esav's. Some people are able to do so.
Malbim (to 27:11): People can err between similar matters, but not between opposites. Their voices were different ("ha'Kol Kol Yaakov"), but similar enough that one could err (so Yitzchak could believe that he erred about this). One cannot err between a hairy man and a smooth man.
Why did Yaakov describe his actions in the eyes of his father as "ki'Mesa'te'a" (rather than 'ke'Ramai')?
Sanhedrin 92a: To teach us that someone who changes his words (who lies) is akin to worshipping idols. 1
Malbim: He will think that I desire [Berachos] for this world, which shows that I despise the Berachah of the world to come, and stray from the proper path.
Ha'amek Davar: He knows that my Midah is a pure man who sits in the tent. If he sees me act like a trickster, I was To'eh (erred, strayed) from the path, which is worse than one who was crooked from the beginning.
Our Sages derive from the word "Mesa'te'a," that one who deceives is as if he worshipped idols. Why is this so?
Maharal (Chidushei Agados Vol. 3, p. 184, to Sanhedrin 92a): "The seal of HaSh-m is 'Emes!" (Yoma 69b). One who deceives (lit.' switches his words,' even if not an outright lie) attaches himself to Avodah Zarah. Mesa'te'a is an expression of "Ta'us" (straying). 1
Maharal must mean that the letters Tav and Tes are interchangeable. (CS) Also see Maharal (Nesivos Olam, Nesiv ha'Emes Ch. 2).
Why did Yaakov need to add the words "v'Lo Berachah"?
Seforno: He meant that even if, after blessing Esav, his father would have reserved a small blessing for him as well, he would lose that, and earn himself a curse instead.
Malbim: If I am passive, I will get a Berachah for the world to come. If he catches my ruse, I will be cursed in this world and lose my Berachah for the world to come!
Why did Yaakov say "Ulai father will feel (and know that I seek to trick him)"? One says "Pen" for a possibility he does not want to occur, and "Ulai" if he wants it to occur!
Ha'Kesav veha'Kabalah: Yaakov, Ish Tam, did not want to trick his father.