I don’t know about macbooks but in most non-apple devices you can configure Mac passthrough in bios, which would overwrite the dongles mac with the mac of the client. Maybe there is an option for that

You simply have to get two managed switches that support VLANs and setup a trunk port to connect them. That’s not difficult at all.

You have one pipeline to the Internet that is 100 Mbps wide and it doesn’t matter if you put the split of this bandwidth in your router or your switch. At some point all your systems will have to share this 100 Mbps between them.

And this shares are dynamic. If one device currently only uses 10 Mbps, the remaining 90 are free to be used by all the other devices. (highly simplified)

And actually, if your router has more then one lan port, chances are high that this is a switch within your router.

Depends on the situation and distance. If possible, get a Cat6 cable from a reputable brand like monoprice and connect it to your router and PC. If you have some budget to spare, you also could try a flat cat cable to make it easier to get it through doors or under a rug.

Yes it is possible and absolutely viable as a starting point. You might will start experience some drawbacks of hosting on a Windows desktop system more sooner than later. Reboots due to system updates as example.

Use it, and think about getting an use office PC that can hold your drives.