Is the macro necessary? If you have ‘Enable Solo on Selected Track’ preference set (Editing->Project & MixConsole, which you can assign to a key command) you shouldn’t need more than that, if I understand correctly what you’re trying to do. Clicking on any event on a track (not just the header) will select it.
And simply because it has just occurred to me that it might be relevant at some stage, if you’re a-b-ing mixdowns, set up a Compare Project - based on the actual project, to preserve markers, etc, but stripped of all else - and use the same technique to see how things are improving (or not…)