We are still considering this feature for future releases, and will continue to update the status when it changes.
The team is working on #2 (and has been for awhile). We will update as soon as we have more news.
ThanksMarco Janssen commented
Really missing the ability to add non-Office365/Microsoft accounts as guests
We acknowledge the customer demand for Private Channels and have been hard at work on it. Private Channels is currently in preview with a select group of customers. We expect to release the feature to the public later this Fall, and will update here when we are starting to roll out to all customers. We appreciate all the feedback you have provided to us on this forum and thank you for your patience.