1)
Why does the Pasuk see fit to mention that HaSh-m plagued Avimelech, "Al Devar Sarah"?
2)
Why does the Torah reveal which plague came upon Avimelech, but not which came upon Pharaoh?
1.
Moshav Zekenim: Sarah gave birth after Avimelech took her, so it needed to teach that HaSh-m had closed all openings, lest one think that he impregnated her. We learn that HaSh-m plagued Pharaoh similarly.