Well, you are basically punching a hole through the Shadow DOM semantics that LWC provides for you. LWC's native Shadow DOM parity is a work-in-progress, but I think it's close enough that you might not see any difference in browsers that have good Shadow DOM support. However, I would assume that things will break in browsers that lag in this area (did you ...
There's a potential data skew problem that could occur with the "generic account" you associate all your cases with as the number of cases in your database grows. This could become problematic for you when you need to perform updates or other actions on your database of cases. How problematic this will become will largely depend on your record sharing model ...
Community Self registration is meant for external users as indicated by the option "Allow external users to self-register"
Also as per documentation:
You can only select profiles that are associated with the community. If the selected profile is removed from the community, the Profile resets to None.
Now, even if you have Salesform Platform user as ...
When a role is created in the Org, a corresponding group related to the role is automatically created. You don't have to create it again in your test class. You can just query it as below,
Group grp = [SELECT Id, Name, Type FROM Group WHERE RelatedId = :role.Id
AND Type = 'RoleAndSubordinates' LIMIT 1];
Hope this helps you