I’ve tried uninstalling and reinstalling both the theme and WPtouch – no luck. Are there possibly some hidden files that I can remove via FTP in order to ‘start from scratch’, without actually rebuilding the entire site over again?
Yes, I had. It worked and looked fine until switching to the child theme. I don’t know if it makes a difference, but the child theme was created automatically by the plugin “Advanced Twenty Seventeen”. It carried over all my other settings without any problems.
One possible interference is that I used the advanced settings to change the background color in the header, which might have messed with the WPTouch settings. But I don’t know if that has much bearing on the initial problem of the customizer not loading.