I'm brand new to SW, but have been modeling in various other software for over 10 years. I cannot for the life of me I cannot get two parts to mate properly. I'm gonna get terminology wrong, so feel free to kindly help out along the way. Here is the sequence of how I build this project:

1. I started with an assembly, and then created a main sketch living in the root is called "Idea". This is the main idea for my drone and I'd like for it to be available throughout all the different parts I design to use as a reference guide. 
2. Inserted a new part and added the "Base" part. This is the bottom plate of the drone. Created a sketch on the top plane and extruded it up by 6mm.
3. Inserted a new part and just clicked on the green arrow without selecting anything. Then I selected the top surface of the base part and began to sketch an arm. The arm will be extruded from the top of the base's surface so it was the best place for the sketch to go.
4. After sketching, I extruded that by 6mm straight up. Now I have the arm in the perfect un-folded position.
5. Floated the Arm part so that it can move 
6. The design idea is that the arm will swing out, so I'll need to design a snap detent in both the open and closed positions. 
7. I now need to "fold" the arm so I can build a stop/detent mechanism for the folded position of the arm.
8. Mate#1: I make a concentric mate with the cylinder of the hole and the cylinder of the arm. This locks it to the rotational axis, but it can still move up/down. 
9. Mate#2: Coincident mate between the upper surface of the base and the bottom surface of the arm. This constrains the arm from going up/down and not it rotate exactly like it's supposed to. 
10. Mate#3? Now I need the arm to be rotated parallel with the base's (or assembly's) right plane. This will be the arm's folded position. All I need is for the one face of the arm to be parallel with the right plane of the base/assembly. When I apply this constraint, the arm momentarily moves exactly where it should go, then snaps back to it's original state. I also tried using an angle mate, which doesn't work either.

Some key points:

• Everything was extruded "normal" or "straight up" ie not drafted, so there's no way that these two surfaces are not parallel to each other. 
• If I design these two exact same parts in another software, export the .steps, bring them into SW into a new assembly, the constraints work just fine. 

So this means it has to do with the fact that I'm referencing the geometry of base part from my arm part. At least that's what I think it has to do with. 

Any guidance would be great thanks. I've uploaded the exact file to this post.

[UPDATE] I have since figured out that converting entities and constraining my sketch to them will always "lock" the part according to those sketch constraints. So how do I go about still referencing the geometry/planes/sketches of other parts, while still maintaining a floating part that I can mate/move later?