Welcome! SmartThings is a very powerful platform, but not very intuitive. As for your specific questions:
It depends on what you mean by “control your hub.” There’s a developer tool, the IDE, which runs in a browser and let you access your cloud account information, but you don’t use that to toggle devices on and off.
There’s also a very popular third-party dashboard called smart tiles. It has customizable dashboards and is a very nice Graphical interface. Runs in any web browser, so you can use it on a PC or on a tablet. A lot of people use a wall mount tablet with it. Definitely worth looking at.
www.smarttiles.click
There’s even a thread in the forum where people discuss various hardware they’ve used with smart tiles.
But there’s no PC equivalent of the official SmartThings mobile app.
.2. Here’s the custom code FAQ. It explains how you can install smart apps or device type handlers that you find in the forums.
.3. Community developers can submit their custom code to smartthings for eventual publication in the marketplace section of the official app. But in some cases this is taking over a year and it’s not always clear what the publication requirements/issues are.
.4. No one knows, and I think that probably includes SmartThings staff.
.5. You didn’t ask this, but you might find the following helpful.
