Amenities space

Hi Allen, I would probably include the core toilets if they are the more likely to be regularly used by your FTE, just as we should if we have an office tenant space with core toilets outside the leased area.  For the fitness/locker fixtures, I would assign a lower % of users based on what's appropriate for your project.
With both of these aspects, include a short narrative for the reviewers explaining your values. For counting the office tenants as visitors...that's interesting. I can see it going either way. The ID+C Ref Guide does advise to not count retail customers in the calcs, though, so I'm not sure there is a clean answer, depending on how you see your visiting tenants. It'd likely be best to email LEED Coach and ask for this aspect. It wouldn't hurt to spell out your approach for other aspects above, too, while you're at it, and get clear guidance.
Please do post back with your decided approach if you get a chance!

One additional note: I've had projects where the core restrooms were outside the boundary, and were not beneficial to include in the credit calcs. For ID+C projects, you meet the Prereq by showing compliance with the fixtures that are within your project scope. If you want points under the Credit, that's where you include all fixtures anticipated to be used by the occupants. I've had existing buildings with older core fixtures, for example, where we were not eligible for any points under the Credit, so I showed savings for the Prereq only and did not pursue the Credit at all.