Hi Kevin,
We are a small General Contracting firm. At this point all we are trying to do with the sharing is allow our subcontractor to see a folder for each job with the plans in the folder. Sharing internally hasn't been an issue for us. Externally we are only trying to share on a read only basis to view plans. We just started using 365 about 6 months ago and have had the issue the whole time. I'm not sure what you mean in the 3rd question. We create a plan folder in each job folder and we are trying to share that plan folder only. We don't need external users to see any other folders.
Thank you,
Ernie