The Sheriff's first wife has been dead a long time, I guess in the crash in the Everglades back in the 90's that he mentioned at the end of the show.
His current wife, the blonde doctor character Mariel, is one of the "survivors". She's also the one that mentioned losing her wedding ring a couple episodes ago. She's also the ranger's ex-wife.
I believe that was the same episode where the corpse that Dave had in his trunk disappeared and all they found was a bunch of sludge and a wedding ring.
They resolved that mystery pretty quick.
Whatever they are they are shapechangers rather than victims of possession.
As far as regular replacements so far we have the sheriff, the doctor (the ranger's ex-wife), the priest, the head of the tv station (the reporter's boss), and now apparently that blonde kid the sheriff's daughter was messing around with a bit.
Dave (the funny alien conspiracy nut) and the son of the ranger and original doctor were almost replacements, but in both cases they escaped and survived.
The airforce captain the ranger found was attacked, apparently escaped after being wounded badly, but died.
Maybe from his wounds, maybe not. He was alive last time we saw him in the scene where the sheriff calls in the priest and tells him to perform last rites telling the priest "he's not going to make it through the night."
Oh and as far as what "they" are, my guess is a deep sea race of highly morphic creatures dredged up during the hurricane. They are obviously aquatic initially. They use luring lights similar to other deep sea life forms. Plus the blood taken from the replacement doctor had an extremely high hemoglobin count similar to what is seen in marine animals such as dolphins according to the show.
Oh and in addition to the lights, strength, and morphic abilities they apparently they have wicked claws since they found that one imbedded in the skull of the airforce pilot.
Ah I believe there were a few gooey tentacles wrapped around the corpse in the trunk.
I'm picturing a glowing jellyfish/octopus thing with hooks or barbs at the tips of their tentacles.