What are the implications of the words "Vesharatz ha'Ye'or Tzefarde'im"?
Oznayim la'Torah: It implies that the Nile will actually produce frogs 1
Why did the plague of frogs begin specifically with Par'oh?
Oznayim la'Torah, citing Midrash: They bit him wounded him and castrated him. See Oznayim la'Torah.
The plague of Arov too, began with Par'oh, as the Pasuk records later - in 8:17.
Rashi: See Sh'mos, 1:9. However, this seems to clash with the Midrash that initially, Par'oh refused to listen to the wishes of the people to enslave Yisrael, and it was only after they deposed him for three months that he relented.
Why does it mention that the frogs will enter the kneading troughs?
Hadar Zekenim (citing Pesachim 53a): This is said after ovens, to teach that they entered the ovens when the kneading troughs were placed next to the ovens - when the ovens were hot. 1
To teach us a lesson in Mesiras Nefesh
Here it mentions Par'oh's slaves before his nation, and the next Pasuk reverses the order!
Moshav Zekenim: This Pasuk discusses the order of entering the houses; they first came to the slaves' houses. The next Pasuk discusses frogs entering their bodies. He and his nation were stricken before his slaves, since they were in the plot to enslave Yisrael. 1
Moshav Zekeinim: As the Pasuk wrote in 1:9 "va'Yomer el Amo".