Do linkwarden instances federate, so that it can act as a decentralised way-back-machine?

No, but openwrt seems to be available for some of their models if that is your question.

I think it’s best to only buy routers supporting openwrt in the first place and switch firmware to openwrt asap. Openwrt or Opnsense or anything open source and well maintained will guarantee security updates years and years beyond the original manufactures firmware.

Ditto.

There is no need fire up a dedicated machine to do this. Use your router/ap running openwrt and connect a hdd via usb. The machine needs at least 128 Mb RAM (256 mb would be better). Install the transmission package, set it up, add a gig of swap space on the hdd and you are good to go. The AP runs 24/7 anyways so there will be very few extra power consumption. Vpns often don’t allow port forwarding (mullvad has stopped support recently if I remember correctly). You can just be a passive node and not often ports, that should work good enough. Consider seeding parts of sci-hub. it’s a project worth supporting imho.

You can just download once of the parts below with less than 12 seeds and set it to host without ratio:

https://phillm.net/torrent-health-frontend/stats-filtered-table.php?propname[]=seeders&comp[]=<&value[]=12&propname[]=type&comp[]===&value[]=scimag

Maybe try the searxng matrix support chat, I think there might be a few developers around that could point you in right direction:

https://matrix.to/#/#searxng:matrix.org

They also have a Mirc support channel is that is more your cup of tea.

I had this problem around ten years ago where I wanted to be able to disable the esp (which prevents powerslides) in my VW Caddy. I found a hardware/software kit on aliexpress that went for around 30$. Maybe have a look there, I don’t know how new your car is and what model it is, but if somebody made an aftermarket kit for your car, there might be a cheap copy of that available from china.

Mine worked fine, I could disable the annoying option where the car always gives 3 consecutive turning signals, no matter how short you move the lever. Also daytime lights could be enabled, and whatnot.

Turning off the esp was not even an option with this board computer unfortunately so I had to interrupt the power line of a sensor with a switch to archive that, which I found out on an obscure German forum thread years later…