SingleEmailMessage

When setOrgWideEmailAddressId is not set, sender will be excutor.

Not Admin User, How to delete/insert FinServ__FinancialAccount by Apex

Apex should be "without sharing".

Remove sObject.SObjectType.getDescribe().isDeletable() or sObject.SObjectType.getDescribe().isUpdateable() or sObject.SObjectType.getDescribe().isCreateable()

Inactive record in FinServ__RollupByLookupConfig__c, if need.

To Delete

Before delete, update "OwnerId" to current user. Like: FinServ__FinancialAccount.OwnerId = UserInfo.getUserId(). Then user can delete FinServ__FinancialAccount record.

To insert

If you want the special user can edit this FinServ__FinancialAccount record, don't forget to update "OwnerId" to the special user's id after insert.