Why did the plague of frogs begin specifically with Pharaoh?
The plague of Arov too, began with Pharaoh, as the Pasuk records later (See 8:17).
Moshav Zekenim (29) citing R. Moshe: What was so bad about being stricken first? They also left Pharaoh first! (Perhaps one feels worse when only he is stricken. Also, they left him first, because he agreed to let Yisrael go! - PF)
Why does it mention that the frogs will enter kneading troughs?
Hadar Zekenim, from Pesachim 53a: This is said after ovens, to teach that they entered the ovens when the kneading troughs are by the ovens, i.e. when the ovens are hot.
Here it mentions Pharaoh's slaves before his nation, and the next verse reverses the order!
Moshav Zekenim: This verse discusses the order of entering the houses; they first came to the slaves' houses. The next verse discusses frogs entering their bodies. He and his nation were stricken before his slaves, for they were in the plot [to enslave Yisrael]