The portal/sharing/rest endpoint is for the home app and for developers who want to extend, automate, or interact with the Portal outside of the home app. It's available for administrators to use, but it does take some getting used to. Everything would ideally be done in the home app.
The issue in your first bullet is that you're likely not signed in. The issue in the second two bullets are that they aren't valid URLs, (there's no datastore resource).
Navigate to https://MACHINENAME.local/portal/sharing/rest and then sign in at the top right. You should be in as an administrator. Then, navigate to https://MACHINENAME.local/portal/sharing/rest/portals/0123456789ABCDEF/servers/. From there, click on the Server ID and update the server URLs.
You'll then need to update the individual items that were brought over automatically during the federation. This requires you to be signed in as well:
https://MACHINENAME.local/portal/sharing/rest/content/users/<admin> brings you to the administrators content page. Click on the links for the content you have and then at the bottom, go to Update Item. Update the URL of the item, (likely starting with localhost), with the correct URL.
Another option, instead of all of that, is unfederating your Server and re-federating it, using the correct URLs during federation.
Lastly, I suggest you look into the ArcGIS Enterprise Builder, which simplifies the setup of ArcGIS Enterprise on a single machine.
