Try to be more consistent with naming
I'm seeing camelCase, PascalCase and snake_case all mixed together. Pick one style and stick with it. Furthermore, I see redundant things in names like Data_type_T, inconsistent use of the underscore suffix for private member variables, sometimes even using an underscore prefix which you should avoid.
I would suggest ...
These items won't necessarily address Excel's memory usage. I think you'd have to have a chat with MS's Excel development team to really address that. This will, however address some of the general coding issues I noticed that should help us write easier to understand and maintain code.
Danger Will Robinson, Danger!!
Debug.Print Now 'Don't know what it ...
Answers to your questions
Am I causing any memory leaks, and is there a better, safer way to do things?
Yes, see for example your second question. A safer way is to use something like std::unique_ptr to manage pointers for you.
In remove_front(), should I set head to nullptr, or should I delete it instead?
You should delete it of course. If you just set ...