The answer is probably 'no' but I want to make sure. For endings like Whiteboard and Broom Closet, is it allowed to also do another ending alongside them? For example say I do the Broom Closet ending, do I have to reset or can I continue onwards and do the Freedom Ending?

I've asked this before and the answer is no. Only ending you can start in the others is pushing buttons for Heaven. So yea after each ending you have to reset.

I assumed so, cheers