Not sure what you mean by pairing with the Sonos One. I’ll assume grouping. But anything fed to a line-in can be played on any and all speakers in the system. Line-in is just another source for the system.
You could prove this to yourself by connecting your phone directly by cable to the line-in.
This article and the various other articles linked within it should give you all the info you need.
https://support.sonos.com/s/article/1080?language=en_US
Edit: my reply expanded at same time @ratty posting his.
I guess the ‘pairing’ is actually grouping.
Line-In Autoplay -- if that’s what’s being referred to -- can maintain the group if required. There’s a check box.
As for the Bluetooth question, it depends what you want. Receivers come in various flavours, depending on things like the audio codec support (AAC, aptX, aptX HD, etc), output capabilities (analog, digital), whether you need a built-in battery, range, Bluetooth version, and so forth. You shouldn’t go wrong with an Avantree.