Alex, I think it would be a waste of money to have an arborist look at that tree, too. You'll have to pay him and then also pay for remedies for the tree.
Spend a little money on some good mulch - at least 6in. of wood chips or something similar - for it this fall after you have moved all the ferns and hostas away from it. Don't plant grass under it, because everything you plant within the root zone of the Wahoo is going to steal water and nutrients from it. Don't plant anything for about 10ft. around it. Mulch generously and it will look nice. Once the Wahoo takes off and is happy, you can gradually introduce a few shade plants back there.
Next spring spend a little bit more of the money on some fertilizer for it, and then make a note (set a reminder in your phone) to water it regularly through the growing season next summer. Don't forget that as the weather gets warmer and the days get longer plants need more water.
I'll bet with all the competing plants removed from underneath it, some mulch to protect the root system, plant food next spring and regular water it will grow and be beautiful.
I agree with you not to stake it, too. It's supposed to grow as a large shrub or a multi-stemmed tree so there's no use staking one trunk. It will just grow others once it is happy. Especially when they are situated on a slope, trees can look really dorky if they are growing too straight.
It sure would help a LOT if you could post pictures of your plants for us.
