1. External access for PowerApps Customized Forms in SharedPoint List: we have been working to unblock this scenario but discovered additional work along the way. Approximately, we are looking at the first quarter of 2019 for this to work.
2. For external access all up: we aim to enable that as part of our 2019 roadmap. It maynot be a nice white-glove service at the beginning but will support the need. Stay tune and hope to share more updates in the new year.