Hi Lynx,
* How can I be sure that the standalone version is the one being implemented?
It depends on what your $PATH is set to. On Linux/macOS, you can run $ type -ap context to list all the ConTeXts in your $PTAH. The first one in that list is the one that will be ran by default. If you want to always run the standalone version, then you'll need to make sure that your ConTeXt directory comes before your TL directory in your PATH. On Linux/macOS, you'd add something like export PATH="/path/to/context/bin/:$PATH" to your ~/.profile; on Windows, you can use the "setx" command. Googling something like "add location to path" will get you more specific instructions for your OS/shell.
* How do I update the standalone version
Just run the installer again. On my system, I run $ cd /opt/context $ sudo sh ./install.sh
(I assume that, as of today, it is already the most up-to-date version
Correct.
if and when it is required or desired to do so?
You only need to update ConTeXt if a new update fixes a bug that affects you or adds a new feature that you want. Most updates are announced on the list, so you could just update whenever you see a new update. Or you could just try updating once per week. You could also just never update. Up to you. The general recommendation is to save an old copy of the ConTeXt folder before you update in case something breaks. I've never done this before though since I can usually patch the bugs myself, and I can always fall back on the version in TL if something is really broken. -- Max