I'm not familiar with the hiseeu brand. What I'm saying here is probably correct, unless the hisseu NVR works very differently from what I know.
- If you want to use the POE ports on the NVR, you must run a separate cable to each camera. The NVR will assign an IP address to each camera automatically. The cameras would not be connected to your local network. You would then connect a single cable from the NVR to a port on the router if you want to reach the NVR remotely or from a computer on your local network.
-If you want to run a single cable from the 3rd floor down, you need to have a POE switch up there. You would not use the NVR's POE ports at all. If you want access to the NVR or cameras remotely or from your local network, you'd connect the POE switch and NVR to the router. Otherwise you could just connect the POE switch to the NVR and have a little isolated network (I've never set it up this way, it seems like it should work).
- Similar to your idea of the NVR in the attic, my NVR is in the crawlspace and I use a wireless mouse and wired monitor connection to it. I do, however, do all of the viewing and management from a PC connected to the network.
You may very well need to learn some basic networking to determine the best solution. Things like what switches to, static IP addressing, dynamic IP addressing, as examples.