Gregrwoodway , a few things here:
1. For Teams calendar app in Hybrid, we recommend Classic Hybrid Config not Modern (Hybrid Agent)
2. The error in Hybrid Agent registration is when you have another Enterprise App or Hybrid App published with the same url. If we are talking about Hybrid Agent app, one way to fix this: if you have the previous HCW logs in user's %appdata%, there is a CC file, retrieve the previous ConnectorApplicationName from there and set it in the current CC file Connector Application Name (backup the file before doing it). More info on HCW logs here: https://techcommunity.microsoft.com/t5/exchange-team-blog/modern-hcw-hybrid-agent-troubleshooting-like-a-pro/bc-p/1573206
3. Usually you don't need to set anything in Internal/External URLs for Autodiscover Virtual Directory. It is all DNS queries, so as long as you have your public DNS records fine and Autod V2 redirecting properly, Teams should be OK with Exchange Autodiscover.