Spoiler alert, but those who care about the story already finished it and those who don't won't read a wall of text.
Theory A: All Nikkis are intrinsically connected to the sea of stars, even ones that aren't originally from earth 1 (our Nikki). This implies that the infinite Nikkis from infinite destroyed worlds aren't necessarily our isekai'd Nikki failing miserably forever, but can be separate alternate Nikkis of different origins. We will rescue Artist Nikki from the comix zone and she will learn to be cheery and happy.
Theory B: The painting is a magic object that works on dream logic, everyone recognizes Nikki as their Nikki because otherwise the dream falls apart. There's no identity theft, just a role to fill in a story about some dragon. The flashbacks seen when finding the scroll were just generated on the spot like in dwarf fortress.
Most likely resolution: nothing really explained, the missing Nikki or why the painting was there or what the time shifts are about, all loose threads to sell you a new island-locked outfit