TextLayoutFormat.fontFamily is null for a selection containing multiple fonts

That's by design.  The property is undefined when its multi valued.  Comma separated list wouldn't work anyhow as that's actually a legal value for fontFamily - but I understand the point in general.  You probably want an array for each multi-valued property.

getFormatOfRange is calling the underlying Selectionmanager function getCommonCharacterFormat.

This code looks something like this:

    public function getCommonCharacterFormat():TextLayoutFormat
    {      

        var firstLeaf:FlowLeafElement = textFlow.findLeaf(absoluteStart);

        var lastLeaf:FlowLeafElement = textFlow.findLeaf(absoluteEnd);
        var leaf:FlowLeafElement = firstLeaf;
        var attr:TextLayoutFormat = new TextLayoutFormat(leaf.computedFormat);
       
        for (;;)
        {
            if (leaf == lastLeaf)
                break;
            leaf = leaf.getNextLeaf();
            attr.removeClashing(leaf.computedFormat);
        }

        return Property.extractInCategory(TextLayoutFormat, TextLayoutFormat.description, attr, Category.CHARACTER) as TextLayoutFormat;
    }

For your specialized behavior I'd suggest writing a new version of this function that takes a textFlow, an absoluteStart and an absoluteEnd as parameters and makes the calculation.  Instead of returning a TextLayoutFormat a Dictionary would be more appropriate.

Hope that helps,

Richard