My Dad may have been in the same Signal Company as your father-in-law.
He was a Sergeant, a field radar specialist, landed D+1 with his squad and equipment, but only took scattered mortar fire going in.
He fought at Saipan, too, and was scheduled for Okinawa, but was pulled off for training for the main Japanese invasion.
He stayed on Iwo Jima until about June, about the same time that U.S. air operations began to launch attacks against Japan from the Okinawa area.
According to my Dad, fighter escorts were impossible before Iwo Jima - the round trip from all other islands was outside the range of all our fighters.
Iwo Jima became the air base for the P-51 Mustang, as I recall.
I'll guess the P-51 was not stationed there for long, and was moved to Okinawa as soon as the area was pacified.
One more thing - when the Japanese held Iwo Jima, it was a key early warning radar station against B-29 attacks on the Japanese islands.
Some pics of the p-51 Mustangs on Iwo at post #29.