Hi eijer!
Based on my knowledge, Microsoft documentation does not explicitly state if Skype for Business 2019 support Office Web App 2013. But we can ensure that Office Online
Server is supported by Skype for Business Server.
The following table shows the compatibility between Office Web App Server and Office Online Server with SharePoint Server, Exchange Server and Skype for Business
Server.

For more details, you can refer to:
https://docs.microsoft.com/en-us/officeonlineserver/office-online-server
Regarding your second question, you can see if there are two new event logs in the Skype for Business Server 2019. First, an LS Data MCU event (event ID 41034)
should be added, this event will report that the Office Web Apps Server has been discovered. In addition to that you should see another LS Data MCU event (event ID 41032) that reports back Office Web Apps Server URLs. If Office Web Apps Server appears
to be configured correctly and has been recognized by the discovery process you can verify that Office Web Apps Server is working as expected by sharing a PowerPoint presentation between a pair of Skype for Business clients. For more details, you can refer
to:
https://docs.microsoft.com/en-us/skypeforbusiness/deploy/deploy-conferencing/office-web-app-server
Best Regards,
Jimmy Yang
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