I didn't see this in the docs anywhere -
If I want to use CancellationTokenSource.CancelAfter(600000) as a failsafe in case my CancellationTokenSource.Cancel() call never gets called.
Will the CancelAfter() interfere with the Cancel() call in any way?
Nope, no interference. When a CancellationTokenSource is canceled, it's an atomic operation. Either the Cancel will occur first and will dispose immediately the active CancelAfter-related timer, or the timer will be triggered first and the subsequent Cancel will be a no-op.
If you want you can study the source code of the Cancel method here.
