1. we use the best parts, including a reliable 3.5mm connector.
2. Yes, 5K has four multi-functional button for vol+/- and playback control.
3. ES9218p provides the best enough output power and sound quality. Unfortunately, assuming the current battery time and price budget, we don't think additional OPA1622 would fit for the current version of HW design. We'll check it out and keep it in mind for the next product anyway.