Note the following is all my speculation.
Though I do think the name "Canvas" is kind of weird, I think the change to originals was great. Before, featured was a confusing name. You could say "I got featured on the front page!" when you're not actually featured, but when you got on the small recommendations or a banner. Now there is no such confusion- Originals is a good title.
For discover, I believe the new name is simply to further differentiate the titles uploaded onto there from the ones Webtoons specifically edits and curates. "Discover", before, sounded like you could find more Webtoon-supported and curated titles like on Originals, which was not true. Canvas makes it clearer that it's a lawless land where anyone can upload to, the word canvas is kind of reminiscent of deviantArt whose process for uploading most people are familiar with- It's uncurated. I do think, despite that, canvas is not the best word they could have chosen. I don't have a better one though.
Regards to why Webtoons doesn't have a forum and stuff, I think it's not a good thing to constantly equate Tapas to Webtoons. Webtoons doesn't want to be a community site. They don't want people to come to Webtoons to see work from X author, they want people to come to Webtoons to read Webtoon branded comics. They're a company that pushes their own branding very hard, and it is not in their best interest to shift more towards a community-focused landscape. And it's working- Their readers are trending upward drastically and the "Webtoons" app is become more and more of a household name. I bring up Webtoons to basically any of my normal friends from High School and they instantly know what is it.
Webtoons wants to be seen more professionally, I don't see Webtoons as a community-upload site like deviantArt at all. It see it more like Shounen Jump, or any kind of manga serialization, except Webtoons also allows you to upload to their site albeit with a clear line between you and their Original titles. Some of the stuff on Canvas is to put it honestly, either hot garbage or sometimes straight up offensive, containing material Webtoons does not want to promote. They put that clear line between the two titles so readers know which content is curated and edited, and which titles are just some random person uploading whatever the heck they want.
I can say pretty confidently though Webtoons will probably never get a forum. I don't want one either lol, Webtoons is FULL of children who would just be fighting constantly in the forums. I think there is kind of this idealization that if Webtoons had a forum it'd be where all the featured artists and editors hung out, and it'd be a great opportunity to network and get noticed, when in reality it'd be 700 thirteen year olds tearing each other's hair out over what ship they want to be canon in whatever Webtoon : ^ P