Does anyone know if there is a GET API call for getting the scheduled iCInga downtimes? I see on the Icinga2 Api page there is schedule-downtime and remove-downtime, but I see nothing about getting pre existing downtimes.
Hello and welcome.
What you are looking for is the Querying Objects part of the API docs 
With it you can query the different object types of Icinga.
To further limit your query you can use filters
example url and filter
url = "https://localhost:5665/v1/objects/Downtimes"
icinga_filter = 'downtime.author=="maintenance-user" && downtime.comment=="automatic downtime for vm creation"'