We are planning to have several external workers for a project. They are going to use a virtual desktop with which they can access an older MindManager version (2016). Is this scenario compatible with the current licensing terms?

Yes, the software may be made available to external employees on a server. Since there is no general downgrade right for MindManager, one needs to sign a maintenance contract (MSA) for the specific product.

Source: EULA_Mindjet_MindManager_October 2015_as is 2017-08-22