You are correct, you can not create a MWAA Environment using Shared VPC/Subnets. AWS MWAA expects the subnets being used for environment creation are owned by the same account where the environment is being created. As of now, there is no workaround for this issue and you will have to use a VPC/subnets owned by your AWS account.
I will relay your feedback as a feature request to our AWS MWAA product team.
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