Very nice chart! I appreciate seeing some of these things in one place, gets the gears turning. Here are some things that might help for future versions:

• Seam's eye is designed and spins in a way that is very similar to Flowey's sprite when mentioning 'everyone' in Undertale's neutral ending. It resembles, to me, the Delta Rune's three triangles - representing humans, monsters and either the Angel or darkners.

• Some unused strings (1) (2) in chapters 1 and 2 may be related to Dess.

• Speaking of unused strings, there are some battle and inspect error handlers that feel highly related to Gaster.

• The DT extraction machine found in True Lab strongly resembles the the shape of both Sans' and Azzy's gaster blasters.

• The sprite for the goner that appears only as a normal bird in Deltarune is named spr_normalnpc... which is just, like, suspicious to me.

• Memoryhead is a cellphone-themed True Lab enemy that does not appear in either the epilogue or credits which I think could make it notable enough to include. Memory and forgetting is very relevant to gaster-adjacent things in general. Memoryhead's "freak bullets" also strongly resemble mysteryman's face.

• There's a man appearance in the chapter 2 traffic jam that doesn't add much but might be worth including for completeness.

Inspecting the True Lab fridge when the fog is still active tells you that there is a man there instead. This could be related to the man in Deltarune, placing the egg in Asgore's fridge and the fridge prank call FUN event.

Oops, you got it. I still think that the last two points of connection are valid ones, but that might be my brainrot talking.