I can clearly see three differant nuts that can be backed off to get this apart.
One in attached to the PVC trap and two steel ones on the drain tubes.
To get the strainer out your going to need one of these if the strainer
parts are broken off in the drain inside the tub.
Ridgid 342 Internal Pipe Wrench, 1-2 In Cap, 4 1/2 L
If there still there you need one of these.
Ridgid 66807 Faucet/Sink Install Tool, 3/4 And 1 In
There is no way to fix that sub floor from below the foor. The tubs got to come out to fix it.
As bad as it is there has to be mold, the bottom plate in the walls rotted and most likly also the bottoms of the studs.
If you do not fix this it's going to next to impossible to keep caulking around that tub.