Amp Questions: No Wifi

I'm bringing my Sonos New Amp to a cottage that has no wifi for it to connect to.

I'd like to be able to stream Spotify music to the Amp.
I believe the only way to do this is to use the line-in from my iPhone (i.e. dual RCA l/r from Amp to 3.5mm male to iPhone 3.5mm female to lightning dongle to iPhone).

As a side note, I know I could bring a little router and setup a local wifi (with no internet access), but this would only allow me to wirelessly stream the music on my phone, since my phone would be connected to this local wifi and cannot be then connected to the internet (for streaming Spotify) at the same time.

So my questions are:

1. Can someone confirm that the line-in method is the best method for this situation to achieve my goal?
2. In connecting in this way, would there be any compromises in terms of sound quality vs. using the amp as per normal on a wireless network connected for streaming to from the internet, etc.?
3. Prior to leaving for this cottage, is there anything I should be setting for line-in, etc. while I still have the amp on my home wireless network and thus where I can access the Sonos app? (i.e. without wifi at the cottage, I will have no access to the Sonos app once I get there to set it up) . For example, should I be setting the Autoplay feature on? Will it work even though I wouldn't be connecting the Amp to wifi once I'm there?
4. For Line-in Source level, which level is best to be set for an iphone connected in this way? And again, as per question 2 above, would there be any compromises in sound quality/volume?