1)
Why does it mention Makas Bechoros?
1.
Radak: Via it, Yisrael left Egypt.
2.
Malbim: I already recognize Hashem via nature - the wonders of rain and lightning. I do not need supernatural wonders like Makas Bechoros; this was "b'Sochechi Egypt" (9) - for the Egyptians and Pharaoh, who do not recognize Him from the creation and the constant conduct. Pharaoh said "Lo Yadati Es Hashem" (Shemos 5:2)!
