As has been pointed out, there isn’t a single “whale niche.” Different species of whales feed on different things, ranging from krill to pelagic fish to benthic crustaceans to giant squid. And their niches overlap with other species. Krill are eaten by many other organisms, including some seals and penguins. As has mentioned already, it appears that penguins may have expanded their niche to exploit more krill in the absence of baleen whales, and are now declining as whales make a recovery.
Yes, although hunting pressure on most species of whales is now low, allowing them to make a recovery.
This expert oceanographic report Ships 
mentions in passing that as of now the ships in the sea weigh more than the fish!
Hunting pressure is significant across the whole food chain even if it’s now eased off the whales.
My very wild guess: since the dawn of history, whales have had a decidedly minor impact on the food chain. I’d rather investigate the dolphins and porpoises, those no-good fish-eating @#@#.
Several analyses I’ve seen indicate that they have, or had, a significant impact.
Again, I’m guessing that would be the sperm whale and other deep-diving mammals who feed on squid and other animals in the deep scattering layer?
You may call yourself a wevets, but by us you’re a webam.
I’m not sure why you would guess that. The analyses included baleen whales.
Here’s a couple of technical references.
Ecosystem Impact of the Decline of Large Whales in the North Pacific
Ecosystem Effects of Fishing and Whaling in the North Pacific and Atlantic Oceans
I think it’s important to note that sperm whales were the most heavily hunted great whales after 1700, since they offered a lot more oil than baleen whales. I’m not sure what that means for baleen populations.
The populations of Right and Bowhead Whales were greatly reduced, and Gray Whales were hunted to extinction in the North Atlantic. Once faster ships became available, the fast-swimming rorqual whales, especially the Blue Whale, were hunted to the extent that their extinction was widely feared.
It’s usually assumed that the decline in plankton-eating whale numbers naturally increased the available plant plankton and zooplankton for small fish and other critters. Alas, it’s not that simple. It turns out, the whale makes its own food, and oxygen, too!
If whale poop did not float, none of that would would work; the plant plankton needs light. So, now we know the old schoolboy taunt, “You’re lower than whale poop on the bottom of the ocean” turns out to be mistaken.
Colibri has covered a lot of this, but I just wanted to add that this is a pretty good subject for debate. As a practical, empirical matter, I would only identify vacant niches as those observed where organisms have entered a new niche (which had to be there, otherwise how could they have entered the niche) or where organisms have left a niche and nothing has been observed taking its place.
Beyond that, the question of whether an organism could survive off some unobserved mechanism is a little esoteric - we’ve got plenty to work with given the observed niche space that is inhabited by the penguins, whales, wolves, and coyotes that Colibri mentioned, and all the other known species and those we’ll continue to find in the future.
So with a recovery of large whales, would it be possible to see an increase in number and size of their predators? Larger orcas, for example? Or great white sharks getting larger and attempting to reoccupy - in some sense - megalodon’s old niche?
A “vacant niche” is a bit like a “gap in heavy traffic.”
They are ephemeral, and are both created and destroyed by the movements / changes of the adjacent participants. And as such their shape is subject to constant metamorphosis as everything is continuously jockeying for position and advantage. Last of all, they’re defined by what isn’t there, not what is. As such they’re a useful epistemological metaphor, but not a real actual thing you can point to in nature.
There is certainly no force holding a niche in a vacant state or at a particular size or shape. Sorta like an enclosed vacuum, the only thing a vacant niche “wants” to do is collapse as the “gas” of adjacent niche occupiers expand into the ecological vacuum.
Occupied niches are more like vehicles in traffic. A group of interacting occupied niches makes up an ecosystem just like a mob of vehicles makes up “traffic”.
Possible, yes, but mostly unobserved (at least for now) - the best of the large whale recoveries is so far the eastern North Pacific population of the grey whale (from about 2,000 animals left in 1949 to about 21,000 animals today) - they were taken off the Endangered Species List in 1994.
However, offshore orcas (ignoring the resident fish-eaters) have consistently been listed in the stock assessments for the North Pacific at between 300 and 400 animals, with no sign of a trend and significant error bars on the measurement. So if orcas are indeed getting more numerous and/or fatter, we’re not able to observe it in a systematic way, which probably has as much to do with our limited abilities to observe and keep track of animals that live distant from shore as additional complexities beyond the simple predator-prey relationship - no ecosystem really only has two variables to shift.
In terms of larger-sized individual animals, I’d look for increased average sizes, not sizes larger and beyond the upper range of observed size for that species. Don’t look for white sharks to turn into megalodons, just look for them to cluster between the old mean and the current observed maximum size for white sharks due to better feeding opportunities. There are likely other constraints on maximum size in genetics and development rather than just prey availability.