[mothered]

The MAC address of any device on a network isn't going to be leaked to the internet except in certain implementations of IPv6

As stated In my previous post and as you've just confirmed here, the MAC address of a given device "can" get leaked. It doesn't necessarily mean It "will" be leaked each and every time.

The DNS server you'd be forced to use would depend on what the library's guest network forces you to use, unless you configure DNS on your computer to use whatever you configure it to use

You do not understand the concept and Inner-workings of Transparent DNS proxies. I'm trying to educate you, so please take this under advisement.

Irrespective of the DNS settings on your device, If the ISP that you're connected to Is utilizing a Transparent DNS proxy, they (ISP) Intercept all DNS traffic, hence reroute all requests/look ups by forcing the end-user to use "their" DNS servers- regardless of the DNS settings on the device per se. As such, they "transparently" proxy all requests, hence whilst you believe you're using the DNS servers as set on your device, all traffic Is In fact routed via the ISPs.