Good day all! I will try to explain as succinctly as possible.
The permissions/sharing context for the below is as follows
1) Security Controls > Sharing Settings > Default External Access:
Account, Contract & Asset = Private
Contact = Controlled By Parent
Case = Private (cannot be changed if Account = Private)
2) Sharing Set -applied to the Community User Profile in question-:
User:Account = Account:Parent, grant Read/Write to Accounts
User:Account = Case:Account.Parent, grant Read/Write to Cases
3) CRED Profile Settings -applied to the Community User in question-:
Account = Read & Edit
Contacts = Read & Edit
Cases = Read, Create & Edit
In brief, a Customer Community User using a non-Plus license cannot access Accounts via Lookup when attempting to create a Case, seemingly following both the Sharing Set and the External OWD at the same time.
When the User logs into the Community, there are 3 Tabs: Cases, Accounts and Contacts. The Sharing Set allows the User to see only the Accounts (and the Contacts underneath those Accounts) that have the User's Account as their Parent Account. So if my Community User originates from the 'Google' Account, it can see the 'Google Mail', 'Google Drive' and 'Google Maps' Accounts and Contacts without problem because all of them have their Parent Account set to 'Google'. Clicking on Cases shows similar. Only Cases that belong to 'Google Drive', 'Google Maps' et cetera get listed.
Now, when the User clicks on New Case and attempts to create a Case and append it to an Account, the Account Name lookup field provides NO access to ANY Account. Even Accounts that the User can see and edit on the Accounts tab, spelled out with precision. With Account Name being required on a Case, Case creation is of course impossible.
How is this "Can Access But Can't Access" situation even possible? External OWD are invoked when creating a Case, but Sharing Set is invoked whenever reading or editing records??
To make matters even more interesting, if the User first clicks into an Account that it can see on the Account tab, say 'Google Sheets', so that it becomes listed on the "Recent Accounts" list on the Case's Account Name lookup window, the User is SUDDENLY ALLOWED to create a Case appended to that Account!
What must I do to successfully create a Case with this User via Community with the standard lookup accessibility?