There is no reason to think Inception copied Jonny Quest...
... because they're both drawing on much older concepts which are common enough that everyone from Doctor Who to Scrooge McDuck has done it. The "basic points" shared by each are also shared by countless other intellectual properties dating well before Jonny Quest's 1996 debut. Let's look at your points. I'll stick to relatively recent examples as they're easier to find, but rest assured that most of these concepts go back to at least the 60s in their current form, and have their roots in mythology.
Dying in the dream is bad. TV Tropes has a whole article dedicated to this idea. Notable examples include 1984's "Neuromancer" novel, the 1989 "Shadowrun" RPG, and the 1984 Nightmare on Elm Street film. Notably, dying within a dream in Inception does not harm you: instead it pulls you out of the dream. The danger is killing yourself in the real world because you think you're still dreaming.
Dream weaving. The idea of being able to control the environment of dreams is very common, very old, and very real. In fiction, we can find it again in Nightmare on Elm Street as well as cultural touchstones like Buffy and Stargate SG-1. Inception is unusual in that once made, a world is usually difficult to modify on the fly; most versions of this trope have the dream world as utterly transmutable.
Sharing dreams. The very first known version of "Beauty and the Beast" has this. Star Trek has done it several times, and even The Powerpuff Girls have explored it. "Questworld" seems to be more like a shared virtual experience than a shared dream, which is the territory of Tron. Again, Nightmare on Elm Street is predicated on the idea that a person can enter someone else's dream.
I'd like to point out this article which, while less than scholarly, points out that a much closer parallel to Inception can be found in Scrooge McDuck than in Jonny Quest. I'll also mention that the Riddler invented a Questworld-like machine in a 1992 episode of Batman: The Animated Series. See also Total Recall, and a handful of episodes in X-Files and The Twilight Zone.
Does Questworld share similar traits with the conceit of Inception?
Yes, it does, because they're both drawing from the same well: unreal experiences shared by multiple characters, the effects of which can adversely impact the users' real-world bodies. It's a popular trope with roots in antiquity, which became increasingly common in the 80s and 90s.
Nolan was very upfront about his sources of inspiration (see below) and Jonny Quest doesn't seem to have been among them. There's no real reason for him to have avoided mentioning it while talking about his other sources.
In particular, I see no reason to single out Jonny Quest's version as THE sole inspiration for Nolan's film. They certainly don't seem to have enough specific similarities to make the accusation of Christopher Nolan "stealing" Questworld --especially in light of the fact that one of your points (dying in the "dream" is bad) is actually a major difference between the two works.
What WERE Nolan's influences for Inception?
In my above lists I deliberately avoided mentioning anything that I know Nolan has cited as inspiration for Inception.
He cites films like The Matrix, Dark City, The Thirteenth Floor, and Memento (all released from 1998 to 2000) as direct inspirations. Paprika is a 2006 anime about entering the dreams of others, and he claims to have drawn heavily from that. Blade Runner, while not about dreams, is an influence because it shares the theme of being unsure what is real.
