Perpetual Fallback License?

This means that you can use the version you have purchased (2016.1) forever, even after your subscription expires. Our sales team has contacted you with more details.

Can one port the personal perpetual fallback licence on to another computer or use it on both computers at the same time?

Tried to install and use my licence on a new laptop I recently bought, but when Trying to use the license key, it would prompt me it has expired and wont allow me to use the product at all. 

So I wonder if I'm stuck to use the fallback version only at the old computer, which honestly would be terrible.

You can, it looks like you either use a different license or a different product version you don't have the fallback license for, please contact the sales team for details: https://www.jetbrains.com/support/sales/.

So I could use my license e.g. on Windows machine, and also in Oracle VM VirtualBox (installed on the same laptop), if I am restricted to Linux based tooling, which is not working properly under Windows?

Of course taking care to have the same major/minor version according to the license...

We are heavily struggling with Bazel plugin / Java dependencies interaction under Windows, and I don't want to dive into IntelliJ Bazel plugin engineering in order to solve it by myself.
However, I don't even know if it would have been solved with a newer IntelliJ version, as I cannot use the latest plugin due to compatibility cut-off of my licensed version 2021.3. :(

So I could use my license e.g. on Windows machine, and also in Oracle VM VirtualBox (installed on the same laptop), if I am restricted to Linux based tooling, which is not working properly under Windows?

Yes, the same license can be used on multiple machines in parallel if the OS user name is the same on both machines. Otherwise, a license collision dialog will be shown. Please see https://intellij-support.jetbrains.com/hc/en-us/articles/207241005 for more information.

We are heavily struggling with Bazel plugin / Java dependencies interaction under Windows, and I don't want to dive into IntelliJ Bazel plugin engineering in order to solve it by myself.
However, I don't even know if it would have been solved with a newer IntelliJ version, as I cannot use the latest plugin due to compatibility cut-off of my licensed version 2021.3. :(

I'd recommend you discuss Bazel-related problem with the plugin vendor at https://github.com/bazelbuild/intellij/issues