Referring to Server 10.3 and Collector 10.3
1. docs say tracking layer can be an AGOL hosted feature service or portal published feat layer. What about publishing to server, then "add item" on AGOL, just like we do with all our other feat. services? I.e. I don't want to use AGOL credits, I have my own server.
2. can location tracking be collected in offline mode?
1) ArcGIS Online will require it be a hosted service, Portal can use an ArcGIS Server service. To use the Portal location tracking from an ArcGIS Server service... there are some specific steps required to get it working documented on the map package used to create a location tracking layer. These areas of Portal that you will need to go in and change some item info through the sharing REST page which is not exposed in AGOL.
2)Yes Location tracking can be used in offline if it has sync enabled.