Hello! I also run a Mardu Control Deck, so seeing more people playing it is very exciting! I really like your version and definitely see some creative ideas at work. Biggest problem I've found when running my version is my opponent's threat density. If they play a lot of creatures, I can pretty much be sure Nahiri won't make it to 8 loyalty. So I find myself looking for other win conditions and
Lingering Souls ins't usually enough.
I respect the "run no creatures other than Combo pieces" thing, and did the same myself for several weeks. It has the benefit of your opponent having
dead removal in their hand game 1, and forcing them to side it all out post sideboarding. unfortunately, it means their removal will all be saved up to hit your
Shambling Vent when you activate it.
So adding some additional creatures will help the manlands survive longer. Some great options for a deck like this are
Young Pyromancer,
Grim Lavamancer, and
Goblin Dark-Dwellers. They may soak up some Lightning Bolts, but that means less bolts to the face or at Nahiri. And when they do survive, they do a lot of work. And if you end up adding more creatures,
Kolaghan's Command becomes a great card.
If you definitely want to stick to the "no creature" path, I totally get it, and my only suggestion would be a couple mainboard
Timely Reinforcements and an
Elspeth, Sun's Champion. Timely provides good blockers and life gain, while Elspeth can win the game by herself. 3 tokens a turn and a built in big creature wipe is ridiculously powerful.
I hope you find this helpful! Good luck and Happy Hunting