New interpreter error: LOW_MEMORY,MEMORY

Attempted to load three unminified deployed modules with three require() statements.

Gordon has a copy.

Where? You mean the moduleNeopixelInit.js code you pm'd me 3 weeks ago? And you're saying it hasn't changed at all since then? It had a bunch of issues which I'd hope that some of the questions we've spent ages answering over the last weeks would have allowed you to fix.

Still would like to know the threshold

There is no threshold. For LOW_MEMORY it completely ran out of memory and had to delete some of the command history to free some up. You could argue that you get that memory warning at around 50% though, given the command history is basically a copy of what you upload.

how is Espruino handling circular references - through (nested) module requests

Yes, it keeps track - so you don't end up with it filling memory with duplicate modules :)