Well, that's very specific case to be honest. In that case you can just download whatever you want beforehand, from soulseek, for instance. Generally people make preparations in advance for such things.

Zotify works very well at downloading Spotify lists, from playlists to whole discographies. You have to sort the output a little as you'll often get multiple copies of tracks due to remastered editions, songs released as singles etc. But overall it's an incredibly easy way to download music.

1. spotx - well, I tried, but honestly, I don't need one more app especially when everything works in the browser. Plus, I prefer portable setups and there's no portable version of spotx, so, it's pretty useless for me.
2. It's explicitly said on the Spowlo GitHub page that they use YouTube for downloading, so, there's no way to get 320 kbps tracks or flacs. Instead, I'd recommend to try some Qobuz/Deezer-based solutions, there are quite a few of them there. Or use slavart or something like this, or good old soulseek, there's even an Android client for it.

Oh man, you shouldn't be using iphone in the first place, the most useless device piracy-wise.