For 3) As of now only 4 stages are supported in a pipeline, but giving the flexibility to add more stages is on the roadmap. So for 6 environments, creating multiple pipelines and triggering one from another is the only option.
For 4) If you package the entire application, you will only need to do deploy existing artifact with only 1 artifact. Deployment Manager works with the premise that you are packaging the entire application. Here is the recommendation on the same: https://collaborate.pega.com/discussion/how-should-my-application-product-rule-be-configured . If you absolutely need to deploy multiple artifacts then triggering the pipeline multiple times once with each artifact is the option.