Thanks for your assistance, have never even heard of security salts berore... not sure what they are or what i should do about them. However I have managed to narrow down some of the error.
a) it turns out that the problem was with that particular mec shortcode... changing it to a different one seemed to allow the page to preview properly.
b) I created a new shortcode with identical settings to the culprit and voila.. it works fine again.
So being able to edit the content despite previewer not working helped, but using the layout tab only gives access to the rows... the skeleton viewer was the hero for drilling down into the actual elements... allowing me to remove/edit as necessary.
Sadly map issue is still unresolved and i'm not willing to un-install and re-install just for that. I'll work without maps enabled. Clearly there is still an mec issue apart from map not rendering, also social media module, cost module went AWOL too.
Once again thanks for assistance... may be back at some point.