Line-In not showing up on Sonos S2 App.

That’s just the standard ‘grouping’ mechanism. And there should be a 75ms delay between the ‘room’ that is playing the TV input, and any grouped room, as the Sonos buffers the sound so it can be played in sync across all the connected rooms. 

Surely a line-in would be treated the same as the input from the TV.

No need to ungroup/stuff around with speakers.

So many articles out there state that you get a pair of 5’s and then hook up your turntable, but hmm you cant use them for anything else - i.e. enjoying a great movie - without reconfiguring your system.

No, they’re different beasts. A normal line in is analog, and is expected to be streamed in sync across multiple ‘rooms’, so Sonos takes the time to both digitize, and buffer, the sound so it can be synchronized on a 2.4Ghz network signal in up to 32 rooms.

A TV input is digital, and is expected to play within the one Home Theater room only. As there is a need to worry about lipsync with the video, Sonos gives up that buffering for other rooms, and uses a lower latency, but shorter distance, 5Ghz signal for the extra surround and subwoofer speakers. But when you send that signal from the Home Theater room to play in any/all other ‘rooms, the signal has to enter the normal buffering process, so you then get the delay.