R – Do I need to publish the public key from .snk file

The simplest way is an anonymous method passed into Label.Invoke:

// Running on the worker thread
string newText = "abc";
form.Label.Invoke((MethodInvoker)delegate {
    // Running on the UI thread
    form.Label.Text = newText;
});
// Back on the worker thread

Notice that Invoke blocks execution until it completes--this is synchronous code. The question doesn't ask about asynchronous code, but there is lots of content on Stack Overflow about writing asynchronous code when you want to learn about it.

This has been a tricky balance for some time. The issue comes from the fact that you need to distribute your Interop assemblies with your code and you may be signing your own assemblies. If you sign your assembly then all the assemblies it references must also be signed - including the Interop assemblies. So you have to sign them.

If you are distributing a standalone application then there's no risk and you should just go ahead and sign the assemblies to make your life easier.

If you are distributing component libraries, things can be a bit trickier since another developer using your libraries might generate their own interop assemblies but sign them with their own keys. This causes all sorts of naming and dependency issues.

Depending on how complex your Interop assemblies are - you can generate the proxy code into a separate .CS/.VB file and compile it directly into your assembly. Then you won't have to worry about strong name issues.