Maybe I’m just missing something about that motherboard, but:
It doesn’t look like that built-in wifi comes out
The M.2 Connector on the bottom is not the same connect as a m.SATA (see picture in link)
I could be wrong about the wifi, but it looks soldered on just like the rest of the coastline components. The screws are just there to firmly support it so it doesn’t snap off. So to answer your questions
Yes, it is crazy.
No, it isn’t possible with that hardware.
I am a fan of tiny builds from computes to houses and more, but sometimes it simply doesn’t make sense. I’ve never seen a tiny house too small for for a full size PC, so it seems even less pertinent in most other instances. Many of the so-called Micro ATX boards out there are smaller than the 9.6" x 9.6" standard. For instance, I have a board that is 8.9" x 6.7". It gives you a lot more flexibility if you have a little more space and native connectors.
I know, there are thousands of Mini-ITX boards out there with huge graphics cards, and nearly every single one of those builds are a compromise and fraught with issues where it won’t go together the way they thought during their first attempt at assembly. Just something to consider. If you are expecting everything to ‘just work’ then I personally would avoid Mini-ITX + discreet GPU.
There are no mPCIe slots on that motherboard, so you’re going to need different adapters/extensions. The WiFi card is an M.2 A- and E-key I believe.
As for bifurcation I don’t see why it wouldn’t work, but with your parts list I’m not sure what needs x8/x8? I know almost nothing about virtualization, but I know that GT 1030 certainly isn’t going to saturate x8 PCIe 3.0 lanes in normal use.
I think using the rear M.2 would be more viable for the GT 1030 and save the x16 slot for the GTX 1080. The GT 1030 only pulls like 30 watts stock, so it only needs 2.5A at 12V, really not a whole lot.
But yeah, it makes more sense to me just to use an actual mATX motherboard. Cheaper and won’t have to use adapters.
I’m still not sure why the requirement of x8/x8. x8 is the data bandwidth, it has nothing to do with the power it can carry. All PCIe slot power is from a few pins that come before the key notch, so any PCIe slot whether it be x1 or x16 are spec’d to be able to put out 75 watts for each slot on the motherboard.
So using the ASRock AB350M Pro4 as an example, there’s a slot for two GPUs and a NIC. Everything fits and has the requisite PCIe slot, but it comes out to less than half the price of the aforementioned X370 motherboard and adapters. This would theoretically be even better in terms of power delivery to the cards since each card has its own slot and is not sharing the power delivery by effectively putting two cards in one slot.
Hearing bad experiences with that Mobo, and I wanted to challenge myself to use an ITX mobo for passthrough. Bifurcation seems to make more sense for the GPUs and the M.2 adapter is more for the extra NIC. The Define Mini C should provide plenty of room (more room than a Nano S) to work with the passthrough GPUs. I could also get a riser cable to add a more powerful dual slot card to extend the 2nd x8 away to use for the host. If I wanted a RX 570 for the host and the GTX 1080 for the host, Bifurcation would allow that. with extra bandwidth. I’m thinking about room to grow.
I just put in a single slot card for now just to be sure for slot compatibility. If I find out while building (or someone else building it) that 2 dual slots and the NIC work to occupy all the slots, then that’s likely what will be done.