The attached should work, however I would suggest reading the signatures from disc rather than from building blocks as it makes them much easier to maintain. Put all the files in the same folder and create a new document from the template. The list is easily maintained using
Insert Content Control Add-In and while this example could follow your proposal and have numbered signatures that reflect the list position of the selected name, it would be better if you named the signatures to match the listed names as in the attached.