Thanks for the guide!

A tip for opening: I don’t have a heat gun and my hairdryer couldn’t even get the plastic warm, so ended up gently scoring around the glued mid seam with the spudger with the fine tip curve on one end and the hook on the other. You could also use a fine scalpel like the sort used for art & crafts. Just take it very slowly with light pressure because they’re sharp and it’s really easy to scratch the plastic. This got enough of the glue out of the way to use a plastic clamp to gently apply pressure and make a small opening for an opening pick.

Couldn’t find the exact genuine battery in stock in any country anywhere near me & iFixit doesn’t have many spare parts available with shipping to Australia. The replacement I got from eBay says it’s a ZeniPower Z55H XAD CCC Li-ion 3.85V 70 mAh 270 mWh. Same voltage with slightly lower capacity. Works perfectly! Fixed earbud sounds great, charges as it should, and runs cool. I now get 8hrs of battery life with ANC on the AAC codec. A huge improvement.