How can I route audio from Windows laptop to Mac via 3.5mm for simultaneous listening?

Since the two computers appear to be close enough to each other that they can be cabled together anyway (which seems to be what you have been doing all along), then why not just plug your headset into your laptop? Why bother trying to route the audio from your laptop - to the mini - to your headset?

There's no audio input on a Mac other than the microphone. A circuitous way would be to get a USB audio recorder, get a 3.5mm jack to phono adapter, download something like Audacity onto your Mac and then use Audacity's monitor to monitor the audio through your Mac's speakers. Even then, I don't know if it will work because the Audacity monitoring has always been fernickity on my Mac.

With Sequoia you can set the Mac up as an Airplay receiver so if your Windows PC will send audio to airplay then that will work.

The other thing to research is streaming. You might be able to set up a streaming server on your Win machine and play the stream with Mac Music or another player. The VLC player can stream in both directions, I believe. I've no experience with this and I haven't used a Windows PC since about 2008 so I'm not much help. If it can be done then I'm sure someone will be along soon.

Very interesting way to listen to audio.

No Line-In port on the Mac but you can buy an adapter for that. As for audio routing on Mac, Rode Connect is a very good tool for this with very low latency, but you have to have it running for the custom routing to work.