... I want to know if we define some custom fields for customers (like age, location, etc) these can be shared ? I think most appropiate way is the case study "Subsites with shared components data and independent core" but there you forgot to mention what about files... they are shared ...

... filter. E.g.: Let's assume the report pulls a list of classes Filters are: Class Level and Class Location (filter type is select/drop-down) Use case: If user selects "Beginner" in the Class Level filter, refresh the page (easy to do with this.form.submit() in the 'Onchange event' field ...

Please check last two case studies, you can have single Joomla installation:
https://alterbrains.com/joomla-extensio ... se-studies

Content will be same but displayed differently per site via site-specific menu items.

No, it's useless, in any case you will not see anything special - just a usual website.

... 3 or 4 page content layout (modules as well as articles), but for which the actual content can be different on each subsite. Please check this case study, you can have single Joomla website which is loaded via multiple domains: https://alterbrains.com/joomla-extensions/administration/multisites-manager/case-studies/subsites-with-minor-differences-in-menu-items-and-or-modules ...

It depends on what's required from files and database tables point of view. Please explore case studies first.

... and a forum running on 3.2.2. Now, in the Requirements I read 'phpBB 3.1+ doesn't work with Gantry5 Joomla templates.". Is this still the case? If so, what does it mean? Eg. integration of the form into the joomla part will be a no go I assume, but syncing phpbb and joomla users would ...

Please check last two case studies:
https://alterbrains.com/joomla-extensio ... se-studies

Session can't be kept between such domains, but we cave single login.

Note sure if this is possible reading the case studies. I am thinking about one domain and five subdomains. Already figured out how to handle the SEO for the subdomains using a third-party service to generate the sitemap XML. I want to run one ...

You have to create virtual hosts manually, in this case you can load websites via full-feature domain names but not via http://localhost.