Coelacanths did live in fresh water, and some them adapted to the seas, but none were equipped with legs. They had all the bones in place, but they weren't adapted to walking. Some of their close relatives did have legs, and did walk on the bottom of ponds, but even then, it was as long time before any fish had legs robust enough to walk on land.
Many early fish had lungs. Ever see a goldfish gulp air? It's gullet is slightly adapted to absorb oxygen from the air. Some fish have evaginations of the gullet to increase surface area. Those are primitive lungs. A few modern fish have retained well-developed lungs. In most fish, lungs have become mere air sacs to control buoyancy. Coelacanth embryos do start to develop lungs, but they stop developing and are mere vestiges of the lungs of early Coelacanths.
Until they found two modern species that had avoided extinction. As you know, neither of them exist in the fossil record. All the ancient coelacanths are extinct.
As you know, that is a false statement. This isn't the first time you've tried to peddle that story here.
In fact, all but one suborder (now one genus and two species) of coelacanths are now extinct. The order had 6 families with a large number of species. All the many different kinds of coelacanth that existed in the fossil record are extinct.
The fossil record shows that there were marine coelacanths. Just not like the ones we see today.
See above. The lungs of a coelacanth are vestigial, mere remnants of the organ that became a gas-filled swim bladder in some other fish.
Coelacanths maintain buoyancy by a different organ, a sac filled with lipids, which makes them less dense.
Technically, the are lobed fins. They do have a humerus, ulna, and radius bone, but not adapted to walking as other lobed-fin fished developed them.