Well, installing new themes and plugins without command line access can be frustrating, I feel you.
Also, please mind that custom method of adjusting themes and blueprints is rather adding new files than modifying given ones, otherwise you can have ugly surprise after update!
@DigitalStefan:
I did just try to install the ‘sidewalk’ theme, but that refused to show up in the list of themes, despite giving a ‘success’ message.
Did you installed it via FTP or via Admin panel? Did you clear all the cache afterwards?
@DigitalStefan:
I’m sticking with the default theme, but now it seems I’ve got to dig a little to get rid of the Lorem Ipsum sidebar text widget, which annoyingly is not exposed as feature in the admin panel, but hardcoded in a file somewhere (Google is my friend on this).
It is actually pretty simple about template files - when you create any page, you have to select it's default template - see pic:

So id you are displaying this page and seeing this sidebar, it means that this template has it coded or includes some other that does. In this case, as name of Page Template on the picture is 'Default', twig template that you should be interested in template file named default.html.twig
@DigitalStefan:
I’m going to persevere. I just hope usability issues like this are revisited and the apparent dev arrogance disappears.
What are you actually trying to achieve?
If you only want to set up simple blog without diving into theme developement, you probably should've use one of few Blog Skeletons - they exist for this very purpose, to kickstart your project without the unnecessary fuss with developing themes and plugins. Also, before throwing accusations about 'dev arrogance' it would be nice to… read the manual
On the other side, if you wish to learn how to code, use templating engine and develop your own projects with system based on most popular PHP framework - Symfony, you can do this as well.
But then you wouldn't rant, would you?
You really don't have to preserve anything. If you are not feeling comfortably with software, simply don't use it. But please, before you start screaming about how the tool is flawed and devs are arogant, well… Read the requirements. Read the instruction manual.
Cheers!