Is there a way to download this demo so i can play it offline?
Yes. If you use the itch.io desktop client, it's available for download there.
Note that if you also Launch it through the client, I believe the save files will be online in your browser's cache. But the file itself will be downloaded onto your computer, so if you locate the file in your file explorer (which you can do easily by right clicking the Launch button and selecting 'Open folder in explorer'), I think you can open it locally and use local saves. That might not work for all games, though, and I haven't tested it with this one.
Yes, what MarloweC said! I also think that once you open it and leave it open in your browser, it should keep functioning even if the internet is disconnected. It loads as a webpage, so that initial launch should contain all the data in it so you won't need to stay connected. That at least works from my laptop that I know of. I assume mobile would do that same.