1) Both options would work.
If you deploy everything on the GW Hub / Frontend Server then you can use intent-based navigation for a fully integrated Fiori User Experience. UI Deployment on PO is more straightforward, but integration into the Fiori Launchpad would only be remote (via URL). This will have some drawbacks like different UI5 versions, different user language settings etc.
2) SAP Process Orchestration license is based on cores, not users.
Any user accessing the Business Suite system must have an appropriate application license, independent whether the access is done through Fiori/Gateway or through PO.