To resolve the complaints about duplicate account records and data quality issues, integrating Salesforce with the ERP and making the ERP the system of truth ensures that the data in Salesforce remains consistent and up-to-date. Additionally, implementing a de-dupe solution within Salesforce addresses the issue of existing duplicate records by merging them and establishes clear account ownership rules to prevent future duplicates. These combined actions ensure both immediate resolution of the duplicate issue and maintain data quality over time.

To allow the UC sales team to view customer account status, invoice history, and open opportunities without navigating outside of Salesforce, creating a mashup page is the best solution. A mashup page can integrate data from the on-premise web-based billing and invoice system into Salesforce in a seamless way. This approach ensures that the sales team can access all needed information directly within Salesforce without the need to migrate extensive data or create complex custom objects and Apex callouts.

The requirements indicate that legacy inventory data will be stored in a custom child object (Inventory__c) and needs to inherit the same sharing rules as the standard Account object. Additionally, related Inventory__c records should be deleted when an Account record is deleted. The proper relationship for these requirements is a Master-Detail relationship on Inventory__c related to Account. This ensures that Inventory__c records follow the sharing rules of the Account and are automatically deleted when the related Account is deleted.

To address the issues with data quality and reliability in Salesforce, it is essential to first enrich and augment the data with complete and accurate information, which can be achieved by exploring third-party data providers. Implementing a data-quality dashboard would be crucial to actively monitor and correct incomplete or incorrect records. Additionally, leveraging Salesforce features such as validation rules can prevent the entry of incomplete and incorrect records, maintaining data integrity from the outset. These steps will collectively ensure the data in Salesforce is reliable for real-time decision-making and reporting.

To reduce the amount of returned mail, leveraging a third-party data source to update the contact information in Salesforce is the most appropriate solution. This method ensures that the contact information is accurate and up-to-date without placing additional burdens on the sales team or customers. It is a more efficient and reliable approach compared to manually verifying information or deleting contacts, which could result in loss of potentially valuable customer data.