Hi Michaël,
Based on your question, I will explain as follows:
1. Create a new hub with several custom entities and allows access to them to Marketing users?
Yes, we can, provided that the Marketing users are assigned sufficient permissions to new hub and custom entities.
2. Are the Marketing users constraint to use only the Marketing hub?
Actually not, Marketing users have permission to read model-driven apps. If other apps are shared to Marketing users, which created in the environment contains Marketing hub, they will have access to read these apps.
3. Can the Marketing users use the outlook connector?
I'm afraid it is not available for Marketing users to use the outlook connector.
4. How many custom entities can be embedded in the Marketing Hub site map?
There is no limit on custom entities embedded in the Marketing Hub site map. However, if you have Dynamics 365 Team Members license, while customizing the app, you can add 15 additional entities.
Here is a reference link: learn.microsoft.com/.../team-members-license
5. Can the user write / update / append / Append to other records? For exemple a new custom entity created specifically for the business of the customer?
Yes, we can add user into the security role and then assign create/ update / append / Append permissions to them of the custom entity.

Here is a reference link: Security roles and privileges - Power Platform | Microsoft Learn
Best Regards,
Sayen Zhang
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