I know you have a lot of questions, I'll say this: I'm making the underswap mod together with lazybones and our team, but I'm also working on this mod (myself or asking for help with some sprites) here's an example

Old house

Hello everyone Demo 2.0 classic swap is available!!! I hope you like it

V1.001 and version 0

I present to you demo of underswap chapter 1 old house. In this version there may be unpainted sprites on hard mode but don’t worry I’m already busy fixing them

Underfell v1.12.6 finally version bye Underfell Hopeless

This sound is irritating

so this room is ready

Underfell v.1.12.5

MR!Underswap

By MR. B0B/Mark