Being homologous only indicates they serve the same purpose---the ability to breath air. There is no way the nose of a land animal can become a blowhole; there is no way legs can become fins; there is no way a tail can become a flapper.
It is absurd to think a land animal surviving very well on land would need to enter a more hostile environment and become something different. That is natural selection in reverse. Whale evolution is necessary or evolution is exposed for the scientific fraud it is. So evolutionists realizing this invent a way it happened and that with not one shred of scientific evidence.
Both of your questions can be answered by pointing out that there are existing animals which show a continuum of limb forms from hands to flippers. Rats can swim well, and have pretty much paws. Water Voles are more aquatic, but still have fairly normal paws. Otters are more aquatic, and have webbed paws for better swimming. Seals have things that are intermediate between paws and fins, but are closest to fins. Walruses have even more finny limbs (but notice how they are used on land), and a developing tail. Then there's the whales, dolphins, etc. with pretty much fully finny fins. You can't say that paws couldn't develop into fins when we can see a wide range of intermediate forms today. Or, you could say that, but you'd be rather silly.
Secondly you say that it is absurd for land animals to enter the water, because it is a more hostile environment. Well, that depends. For an animal that can catch fish, if there are plenty of fish to eat, but little to eat on land because of too much competition, then fish could be an important part of their diet. There are plenty of land animals that eat fish. E.g. bears. Being able to walk around easily is not much good if you starve to death. Venturing into the water and finding food to eat is much better, even if you can't swim well. Hence, the ancestors of whales may have eaten fish or other food from the water. And because of competition for land based food, there was selection pressure to become more aquatic. And they became more aquatic through intermediate forms similar to those we see today in different lineages.
As an example of a land animal not especially adapted for the water (though a strong swimmer and a tenacious pursuer of fish), consider the Fishing Cat
Fishing Cat It has slight webbing between its toes. But, it's diet is primarily fish. Why wouldn't it hunt on land instead? Quite likely because there is competition from other animals, perhaps even other cats, on land. Meaning that the 'hostile' water is less hostile than land.
