Good to know and Kelson trust me, I was just working off some anecdotal posts and I'm happy to learn that I should be fine.
Not giving-up anything.
The Seagate Central is a budget NAS-like device. Yes, use a short CAT6 or CAT6e cable and connect on root network backbone ... the router or gigabit switch if you have one.
1. A wire will be better. If you must use wireless it should work as long as content is around 720p/DTS or lower. In general, wireless is much slower than wired and therefore the router can only handle so many high-bandwidth wireless streams.
Whichever you choose, that connection is only used to download music and videos to the SMP itself for use on it only (nobody is streaming anything FROM it). Any files like that would be loaded on your NAS (hopefully, hard-wired to your router or fast backbone switch). That's where other devices would play or stream from.
2. Depends on how many discs you have. Synology are a nice NAS. See my posts back in thread on how to setup (as much as possible wired) network. Hopefully at least backbone switch can be gigabit/1000 (that will support multiple concurrent large streams away from NAS at once).
3. So, if I'm understanding correctly, you are going to have to select the best straight file format that is supported by all devices or depend on dLNA. Study this and do testing before deciding and spending months ripping everything.
I have always ripped my DVDs to VIDEO_TS Folders with DVDFab (exact full quality straight file copy). There is nothing to get wrong because there is no trans-coding going on (it's only un-encrypting).
For Blu-Rays I use DVDFab/MakeMKV workflow. I seem to be able to get Subtitles and any Extras if I want them. The dedicated Blu-Ray ripping threads (here on AVS) are a better place to discuss this.
I think the wired port is 100 and the WiFi is N (150).
I really doubt any WiFi is faster than wired 100.
Try an actual transfer and time it. A $20 card and some CAT6-6e cable will upgrade that desktop to gigabit/1000. On wired, if the cable is not too long, you should get close to 1000.
He had a NAS so no need to use SMP as a NAS. It should be end-point output device only.
Main switch and NAS (or file server) should be gigabit/1000. Clients should be wired 1000 or 100. WiFi would be my last choice and reserved for laptops, tablets, and other mobile devices.