Org data does not match with the number of measured employees

HI Ashley_Wilfred, good questions! 

a) Measured employees are the employees that have been issued a license via AAD. The most likely reason there is a big difference between internal collaborators and external is probably because the domain associated with those employees does not match the domain that was present for the employees on the uploaded file and the licensed population. Here is the definition: "Internal collaborators have domains internal to your organization, while external collaborators have domains external to your organization."

b) Yes you can, but the caveat is that in order to qualify as a Manager this person must have at least 10 direct or indirect licensed reports. Please see this guide here: Manager settings for Viva Insights | Microsoft Docs and My team in Viva Insights | Microsoft Docs

To set someone up with the Leader role you must assign them the Insights Business Leader: Roles for Viva Insights | Microsoft Docs