How likely are vpn (mullvad) configs to change over time? Will I end up having to constantly update them, which is something I assume the native app will do.

Secondly, do the wire guard tools include things like a killswitch on disconnect so I don’t accidentally send traffic if I get disconnected (eg when the VPN node I’m connected to goes down for maintenance)

You will speak the version that you learned. You will mostly notice regional differences in the idioms and synonyms of the words you know might be used more commonly in one region vs the other.

For example, in London you might hear people use boot instead of trunk.

But the majority of the words will be the same regardless of region.