The port can be 100Mbit/s, this won’t hurt…it just runs slower during additional software package installation. The network protocols are the same on 100Mbit and gigabit, so they are interchangeable.
You need at least USB2 (or USB3) to flash from. The Xavier happens to have a USB type C connector (there is a type-C to type-A cable bundled with the Xavier dev kit). There is a feature which is new, which I’ve never used, which could help your case. The USB-C connector also has the ability to make the Xavier look like it is an ethernet card (to the host PC) and get around the actual ethernet requirement (it’s still ethernet, but the USB cable bundles the function directly to the Xavier instead of using the host’s ethernet). When you first run JetPack it will ask how the PC and Xavier network is connected…normally it would be between using a router, or direct ethernet to host and using the host as a router…but there may be an option to use the network-over-USB as a kind of fake ethernet card (I’ve never used that feature, YMMV…you probably need USB type-C for that, but I don’t know).
What really counts is that the host runs Ubuntu on a desktop PC architecture (and I believe any recent Mac qualifies as that so long as it runs Ubuntu…VMs are not supported, but if you have a high threshold for pain and are willing to experiment, it might be possible for a VM to work).