Are novellas a growing trend among traditional publishing companies?
Some, yes. I expect it varies by genre – and publisher. And it’s still more difficult to get a novella picked up, because they’re more difficult to make a profit from.
In general, no. Most publishers want novel-length works. If you are in the fantasy/sci-fi genre Tor is doing some things with novellas, but I see no indication they are finding much traction. My best advice its to shoot for longer works, they are what the vast majority of publishers are looking for.
Not really. You can use a novella as a freebie to promote your main work, but that’s more for self-publishing. Full-length novels are better.
A good point that I should have mentioned in my post