The warning is immediately after the Delete this record also option description on purpose. A cascade delete occurs when a parent record is deleted, and the child records are also deleted as a result. In other words, yes, you must engage Technical Support to enable Cascade Delete, which is what enables this option.
The Search undefined error in the flow lookup inputs are caused by the lookup fields not being on the sObject's page layouts.
In your case, you will need to add that look up field to every single page layout (of the source object) that is assigned to your community user profile.
From Salesforce help page:
if the lookup field defined in Field API Name isn’t ...