Plugging straight into the Radio works fine. The only downside is there won’t be any more available ethernet ports to plug in anything else on the robot. If you need to plug in a computer to the network (rather than WiFi), you’d have to unplug something. Usually this is more of a concern at competition where wifi connection isn’t an option, but in 2021… eeh. FWIW there is a methodology where you can bridge the USB port and ethernet ports on the RoboRIO, but I’ve not tried it much.
The PhotonVision Pi image will work with mDNS (UI served at
photonvision.local:5800). However, I’m an advocate of using static IP addresses for everything. Either should work though.
The OM5P-AC is fairly locked down. You can SSH into it… maybe… but I’ve not found much useful on board… maybe someone else has?
The roboRIO can do all of the above (yay linux!), and yes, you can and should do that to help confirm the two are connected up.
That being said, the preferred methodology to debug network setup is Angry IP scanner from the development computer. The assumption is that you have a setup which allows you to talk to the roboRIO, radio, and deploy code (if not, go fix that first). Then, on top of that, use mDNS or the ip scanner to find photonVision using your development computer, and go from there.