Experienced Excel VBA Developer Needed for Stable Add-in with Multi-Select Dropdown (Workbook-Specific)

We are seeking an experienced Excel VBA developer to refine and complete a custom Excel Add-in (.xlam) that adds multi-select dropdown functionality to a specific column — only when used with workbooks containing specific keywords in their filename.

We already have a working pilot version of the Add-in that demonstrates the required functionality which took an hour to create which gives you some idea of the simplicity of the actual functionality required. However, it lacks robust error handling and does not perform reliably across different versions of Excel. Your job will be to use the existing code as a foundation and turn it into a production-ready, stable Add-in with proper defensive coding and compatibility across Excel versions.

Core Features:

- Adds multi-select dropdown functionality to cells in a specific column (e.g., column header "Roles")
- Allows users to select multiple items from a predefined, hardcoded list
- Concatenates selected items in the cell, separated by commas
- Supports:
Selecting and deselecting values
Clearing the cell
Working within Excel Tables or filtered ranges (if possible)

-Add-in should only activate when the filename or workbook title contains specific keywords (e.g., "roles", "permissions")

Key Requirements:
- Developed in VBA as an Excel Add-in (.xlam)
- Implement a multi-select capability for a specific named column within Excel.
- Ensure the add-in is robust, efficient, and seamlessly integrates with existing Excel functionalities.
- Must include robust error handling and defensive code to avoid:
Crashing
Breaking native Excel features
Compatibility issues across Excel 2016, 2019, and Office 365
- The add-in should be production-ready, adhering to best practices in Excel VBA development.
- Should use Excel’s event system effectively (Workbook_Open, Worksheet_Change, etc.)
- Well-structured and modular code with clear inline comments
- Designed for minimal interference with other workbooks or Add-ins

Ideal Skills and Experience:
- Extensive experience in Excel VBA programming and developing Excel Add-ins.
- Strong understanding of Excel's user interface customization options.
- Proven track record of creating efficient and user-friendly Excel solutions.
- Ability to troubleshoot and optimize VBA code for performance and reliability.

Deliverables:
- Final .xlam Add-in file
- Refactored and cleaned-up VBA source code
- A brief developer notes document explaining:
Code structure
How the Add-in determines if it should activate (based on filename)
How the dropdown and selection logic works

- Installation/usage instructions

If you have the expertise to create a sophisticated Excel Add-in with these specifications, I look forward to your proposal.

Attached is a screen grab of the pilot version we made.