Assuming you have at least Kickstart 2.04 ROM's you should have an early startup menu. However if your 2000 has an early model Agnus (8370, 8371) it may not be switchable to PAL. You'd need to upgrade that. The 8372A is a drop-in replacement for most motherboard models that's pretty easy to source (came in most later model A500's/2000's), and would allow you to do the chip ram mod (1MB chip ram instead of 512K chip/512k fast), as well.
https://en.wikipedia.org/wiki/MOS_Technology_AgnusAFAIK shouldn't be a problem using the splitter cable to join the audio channels, but it's been at least 20 years since I've done that. Might want to wait for a better answer, on that one.
As an alternative, I'm sure your local craigslist has plenty of cheap-o stereo receivers that you could hook up for real stereo sound. That's what I always did, back in the day.
