Update font size and leading for the Paragraph style which is based on another paragraph Style

Does the pure ExtendScript code work as expected? For example:

var mydoc = app.documents.item(0);
var paragraphStyle = mydoc.paragraphStyles.itemByName('Insert Actual Name Here');

if (!paragraphStyle.isValid) {
    alert('Not found.');
}
else {
    paragraphStyle.properties = {
        pointSize: 11,
        leading: 15,
    };
}

If this works, then we know it isn't something weird about your document or style, and is something to do with the C++/ExtendScript crossover? That's out of my wheelhouse unfortunately because I don't know C++ or the SDK.

When you say "I will have to insert quotes because its a variable I am passing from C++ to Javascript" I think that means that you are building the ExtendScript or Javascript code as a string from C++. So one approach would be to output the resulting string (code) to a text file and check that runs correctly as pure ExtendScript. This would rule out issues with the string building part.

Also note that

mydoc.paragraphStyles.itemByName(sParaStyleName)

only searches in the root level of the document's paragraph styles. If your target style is inside a style group, it won't be found. You must target the style via its group(s), eg.

mydoc.paragraphStyleGroups.itemByName('Text Styles').paragraphStyles.itemByName('Body Text')

 or, alternatively, iterate over

mydoc.allParagraphStyles

which is a normal Array.