Met "Is dat hem" (soms "is dat 'm?") bedoelde ze "Is dat alles? Verder niets nodig?"

Soms zijn er producten die alleen aan de kassa worden gekocht waardoor mensen soms vragen of "dat alles is" dus dat kan inderdaad in dezelfde context als in het Engels gebruikt worden. :)

One thing I picked up when talking to Japanese people is being less opinionated. In general Japanese people won't have in-depth discussions regarding topics that can be very personal as these may become heated when opinions differ.

Regarding formality, as we're on the internet most people will either speak to you very formally or very casually, either way is fine, we're all equals here!

Tardiness is a trait I have myself, though an appointment I'll never miss, they are just that important to me. :)

Keep at it, I'm sure you'll make lots of friends!