Whenever I try to speedrun Chapter 1, the autosplitter does not work when opening from Chapter 1+2. When opening from Survey_Program, nothing else works.

How can I still have the Autosplitter work for Chapter 1 speedruns?

The auto splitter only works on the CH1 demo and not the CH1 + CH2 demo. You will have to run on the old demo or wait until a new auto splitter is made

how can i get the old demo?

maybe you could get the old demo by putting the site through the waybackmachine on archive.org which im pretty sure wont work. i mean you could just search for some copies of some and see what you find