Music can be played directly with the mobile phone via Bluetooth.
The online services require Internet access and should be able to be realised via a hotspot. To set up a hotspot, however, you need 2 devices (1x hotspot 1x configuration client "mobile phone or computer").
No, you can't. I have digitised all CDs and stored them on a NAS (Synology) and said goodbye to all old devices. Streaming from the NAS works very well. I can recommend this way to everyone.
The speaker can be operated via Spotify Connect as well as via the Bose app. However, grouping several Bose speakers is only possible in the app; after grouping, groups can then also be operated via Spotify connect.