It is wired for PCIe, it will probably work, I have personally had wonderful luck with these kinds of things, but you always risk stupid UEFI vendor code stopping it from working.

I'm in a rush but ive put Ethernet cards in both slots of my optiplex 3080 m.2 slots. The top right slot is vanilla and 4xpcie lanes,full speed. The bottom left is A+e keying and only 1x lane which is enough for 2.5gig Ethernet. This is ultimately what i used. For my prfesene box I added a second card, a 2 port Ethernet card to the top right m.2 slot and made it a four port beast.

The key doesn't correspond to capability. It's about the connector's physical shape. i.e. which side is the lil notch on. B has a notch on the right, M on the left. B & M have a notch on both sides and therefore fits into either connector, but it is limited to PCIex2/SATA speed (i'd guess this won't matter to you as pciex2 is around 4gbps even on pci1). It also doesn't mean the length would fit, that'll be a number like 2230, 2280 etc.

You'd have to check the manual for the motherboard to see if it specifies, but if it's labelled m.2 PCIe it should work I think. If it only said SATA then it would not work.

You can try dmidecode --type slot on Linux, it should list them as available and presumably specify the type to be PCI express.

I think ultimately you'll only know when you try, but these may give you some confidence.

I don't see why not, and for $24 you may as well give it a go.

It is PCIe, so yes.

Yes. I'm doing that with a 7040mff right now.

If the m2 to Ethernet road leads nowhere: you could try one or more usb3 to Ethernet adapters.

Yes but I personally wouldn't run OpenWRT on x86. I would instead pickup a old gateway/firewall device that is supported by openwrt.

Not a direct answer to your question, but assuming you have a switch that supports VLANs you don't need a second ethernet port at all.

It should work, but its kinda one of those things where you won't know until you try. As long as the Ethernet adapter is using an Intel or Realtek chipset, you shouldn't have any issues driver-wise

Do note you can use a device with only one ethernet port as a router/firewall, if you have a managed switch, you just only get half-duplex speeds naturally.

Got 3080 recently, there is one m2 for ssd, second one is for wifi/bt.

My router is literally exactly that running proxmox with pfsense, pihole, and wifi controller VMs. No issues works fantastic, make sure it auto boots in bios and you should have a great time.