Question: For example, I want to play all 58 videos uploaded by a single user.

Obviously I could create a playlist, add each video to it, then play the entire playlist, but is there a simpler way?

Answer: 
Little trick I found is to get a user's unique code/string from their profile page's source. Here's how to do it:

* Go to their user page and right-click -> "view page source".
* Search for "vnd" and copy the code that you find (It should start with "UC").
* Open one of the user's videos. In the URL address, add the following to the end: "&list=" then paste the string you copied.
* Final step is simply change the 2nd character of the code you pasted from a C to a U. Hit enter and the video should reload, and now it appears in a playlist of all the user's videos.

This seems to work pretty reliably, but sometimes does not get all of the user's videos.
If you have subscribed to the user, Youtube TV is an alternative: 
<https://www.youtube.com/tv>

All you need to do is link your account with the app. No apps, plugins, or 3rd party webapps needed.