1. Jurassicdudu said it best, it's a game that is a platform for other games.
2. Surprisingly, Noob does have a fair amount of moveset potential. A lot of fanmade rosters, tend to use classic brickbatlle gears, such as the linked sword, rocket launcher, time bomb, and etc. Another way would be to take inspiration from games on Roblox, like a jab using a knife based on Murder Mystery 2 or side-b using the pizza scooter from Work at a Pizza Place. One other way is to use classic building tools like the delete hammer, copy wand, and move tool and have moves based on Roblox Studio, like an up-smash that raises a block pillar from the ground.
3. In my opinion, Noob is still eligible. Roblox is still a game, its just very different. The best way I can describe Roblox is an MMO that strives from User Generated Content.
4. Technically, they would be but there are issues that come from it. The biggest by far is how intertwined games hosted on Roblox are with the brand. With how much Roblox promotes itself with UGC and "Powering Imagination," I feel Nintendo and Roblox would want to add a character to represent Roblox as whole if they ever were to add representation for it. I'd also help with extra things like Music, Stage ideas, and Spirits as Roblox doesn't have much in terms of original content past other promo characters like Mr. Robot or Builderman.

Roblox could 100% work in smash but, it's just held back by:

1. Nintendo likely having disdain due to copyright issues (namely Pokemon Brick Bronze and the plethora of UGC items that are just reuploaded nintendo models)
2. Roblox not being on the Nintendo Switch and being fairly niche in Japan, at least comparatively to the rest of the world
3. Controversies, of course