Yep, bit of a steep learning curve but don't let that put you off.
Your coming-soon plugin was activated before I could get to look at the site. If you would like to deactivate it, that may be helpful.
You've tried most/all of the debug steps, so ultimately you come to a point when there are no more things to try and rebuilding the site is a sensible option. As I say, rebuild it elsewhere so you don't delete the original non-working site, then repoint the domain name.
When I got a bug I couldn't fix, it took far less time to rebuild my site than I had spent trying to debug it.