Please note that the JBR is a custom build that contains a number of important changes and fixes, and it's not recommended to replace this. For M1 support, it's much better to use an M1 compatible JVM and .NET runtime. Rider 2021.2 is an Intel build, because when it was released there was no M1 compatible .NET Runtime.
You can consider an extensible grammar as a "nested" grammar. You have certain extension points in the "parent" and you can attach a new grammar here.
For example, it would be easy to take a C# grammar and add a new operator, such as the null propagator "?.", since the list of operators is extensible. This wouldn't break anything, as it's just a new token for a binary expression.
Or, you could add something bigger, such as LINQ, by extending C# 2.0's Expression syntax rule with a LINQ query expression.
No, it's always been the plan to open source Nitra. It's come from the team who built Nemerle, which is open source, and the team obviously wanted to continue in this manner. And JetBrains has a pretty good track record with open source - e.g. the IDEA platform that is IntelliJ's Community Edition is fully open source.
We do have a lot of future plans for Nitra, but for now, it's worth pointing out that the products that are cross platform are Java based, while Nitra is based on .net. The project is currently Windows only.
Mono support isn't at the top of our priority list right now. However, the Nemerle binaries work on mono, so it's possible you could build the command line compiler and sample applications under Linux. If you get it working, a pull request would be very cool!
Rider 2021.3 has an M1 build, and makes a big difference. It's currently in preview (https://www.jetbrains.com/rider/nextversion/) but will be released in the next week or so.