I personally think Demon's Souls is perfectly playable without the online. Hell, it and Bloodborne are the most single-player "friendly" games because there are no covenants to grind out for multiplayer (well, BB does have Covenants but very little grinding) and the only one item you'd miss out on without access to multiplayer is one piece of headgear because there's no way to get it in pure singleplayer. Managing your World Tendency for the 100% completion is also easier in the offline mode of the game.Is demon souls still playable without the servers running? I mean technically it is but MP was half the game.
Playing through the game in the order of DeS --> DaS 1 --> DaS 2 --> DaS 3 --> Bloodborne is what I would personally recommend. Sticking to release order for the most part means that there's less chance of getting frustrated with going back to a game without the quality of life improvements that you missed in the more recent title. However, even saying that, I would recommend playing Bloodborne last because it's a very different game and adjusting back to Souls gameplay is a bit weird if you're used to Bloodborne and personally I found that Bloodborne was Miyazaki's masterpiece and going back to Dark Souls series after playing Bloodborne would've felt like a downgrade. But that's a matter of personal taste of course, not everyone prefers BB to Souls.