Jakub Piesik ,

Thank you for the questions! Let me see if I can address most of them. 

Setting the number of monitors is supported GPO or CSP. Please see the example below:

And the supporting documentation:

Manage device RDP redirections for Cloud PCs. | Microsoft Learn

The CSP value for your Configuration Profile is under Windows Components > Remote Desktop Services > Remote Desktop Session Host > Remote Session Environment.

The Remote Desktop Client does support adding multiple accounts by adding "Subscriptions" found under the ellipsis. Please see the documentation here:

Connect to Azure Virtual Desktop with the Remote Desktop client for Windows - Azure Virtual Desktop | Microsoft Learn

I will have to track down the deployment of the RDC app. 

Hope this helps!