(glad you found a solution, there’s always more than a way)
but as you’d do “read-only” anyway it is redundant to create that second account. It would make sense though if both participants would want to write to the same calendar.
You can create subscribe-able .ics urls that Apple devices can consume without handing out any credentials. See these 2 links:
- Sharing calendar - #11 by tcecyk (create the link that you hand out)
- Add calendar subscriptions in iCloud - Apple Support (in german)