Okay, first of all: Watering
Trees don't work on a schedule. You shouldn't water it every certain number of days. What works best is to test the soil with a toothpick of choptick by immersing the wood into the soil and pulling it out. If the soil sticks to the wood, you don't need to water.
I have found that the best way to water is to immerse the entire pot into the sink filled with water. Just enough to cover the pot. Leave it there fore about an hour and then, take it out. Your tree is now watered.
Juniper in Nevada? Well, that's a tricky one. It is more healthy for the tree to be outside but, I would personally recommend that you find a local bonsai club and ask them because I am not proficient on Juniper care in the dessert. Though, it is best to leave the tree outside, and the leaves are probably dropping because of the lack of airflow in your house. But, given that you live in an arid environment, I would recommend trying to grow a tree that is better suited to that environment.
Also, a weekly dose of organic fertilzer such as diluted liquid seaweed never hurts. Good luck with your bonsai! (I would really recommend trying to grow something that is better suited to your climate, you can grow out of climate trees but, it will be a million times easier to grow something that is suited to your area)