If you find that the pulling volume is insufficient to drive progress, then add volume. You could just add another set of deadlifts on heavy day, or do 2 back off sets after your top set of 1x5 with a 10%ish reduction. That would all make heavy day a little more taxing than it already is, so you might benefit from just putting in another pulling variant on your light day as you suggested. You'll have to answer which is better for yourself, but those are some more options for you. In any event, you may want to hold off modifying anything until your sure that more volume is needed. Don't fix what ain't broke, eh?
Include what you want and see what you can get away with recovery-wise. We can't tell you what to want and can't know exactly how much accessory work you can get away with, but if you want to add in some curls or upper back work, I'm sure you won't die.2) Should I be including accessory work, or will the training stress of the program be enough as it is? (And even if accessory work might be good, would it be wise to go a few weeks without it to see how I feel?)
Why can't you do Medium, Light, Heavy? What's the reason for wanting to switch it up?3) Friday would be somewhat better as my 'heavy' day. Would it likely make much of a difference if I modified the program to make it Monday = Light, Wednesday = Medium, Friday = heavy, or would the medium day be too close to the heavy day in that setup. (In which case I guess I'd need to make Saturday my heavy day.)