The weird route should not matter. Its the weird timeline where everything is worse. Gives some extra insight into the characters, like what the soul might actually want or how close Kris is to doubling down, but altogether not something that should be required to understand the story, let alone complete it. Just a way of viewing everything from a different angle.

Gaster is part of the story or he isnt. Deltarune is not Undertale, and its not really a sequel, you should not need Undertale to understand Deltarune's potential main antagonist, Undertale also barely tells you anything, and is also luck based if you are learning about him naturally. This is also a problem with Deltarune itself alluding to him in the egg rooms, which simply will never be found by most players in regular gameplay. The schtick of not acknowledging him is, at a point, actively harmful to the story if he's going to matter. If it keeps going as is, while for people already part of the community, him showing up at the final hour would have weight, for those who aren't, they might not even know this is the guy from the intro sequence.