Best way to setup PowerBI

So I've been investigating a lot last year and trying to pick it up again.
See: https://community.powerbi.com/t5/Service/Best-practice-sharing-customer-specific-dashboards/td-p/323746/page/2

Anyway I'm trying to figure out the best setup for us.

-Distribution to multiple customers

-Custom dashboards per customer next to default dashboards for all
-Multiple devopers

Sofar I was thinking about:

1a. Every developer can use their own powerBI installation, but
How do they share their reports and data with each other?
How do they manage publishing

We could use (are using) one powerBI useraccount at the moment so that publishing will be to the same account, but the PBIX will never be shared between the different developers as far as i know? So how do they take over or continue each others work.

1b. A very simple solution would be to develop every single thing on a server,but then you can't work simultaniously

2. Why should I use PowerBI desktop for creating reports? I can only create one at the time.
I thought this was different before. Has this functionality moved to the online app?

3. On our server we have the scheduler for data installed, should we have PowerBI desktop installed here as well (besides point 1b above).

4. So what is the best publishing way? Is this still:

-Create credentials in our active directory to eah customer.
-Activate Power Bi Pro License (they should)
-Create Power Bi Desktop reports with a rule to filter by customer (RLS). PowerBI app reports I guess?
-Then Publish the report.
-In Service, go to security of the datasource and add the credentials to the rule you have created in Power Bi Desktop.
-Create a dashboard and share it with them.

5. As I'm only able to create one report at the time (with multiple pages) and publish it, how can I download it later when I need to adjust it?