Hi,
Regarding your first question:
I believe they are using Unity 
Had my DK2 my Leapmotion for a while, started by trying to use Shiva3D but found it too limiting/not enough support for the Oculus or LM. The only plugin I did buy for the DK2 in the Shiva store ended up with me getting a refund from the dev (it only supported the DK1 but did not say so in the store at the time on purchase) he has since given up on the plugin) & also a lack of support feedback in their forums tbh!.
I had used Unity before a couple of times (4.1) so decided to go back to where I started having had a break from using Unity for a while. I’m LEARNING Unity (5.1.3f1) based on Leap motion v2.3.1+31549, Oculus Utilities 0.1, Oculus Rift Runtime 0.6.0.1 & 0.7on my other system.
My thoughts experience so far (in bullet points)…
Recent Oculus updates seem to have left me with a few issues to resolve! Mainly either 'Sync cable disconnected error msg in Oculus Utillity (even though is physically connected & tripple checkd!) and I also have had the other msg 'tracker not attached…
See Forum thread(s):
So between these problems I have been chasing my tail upgrading/downgrading software/runtimes ETC in an attempt to resolve them. I am now at the point where I will just live with the sync cable not attached error msg as my DK2 seems to be working with games/demos/Unity etc.
I had LOTS of crashes/BSOD issue on one of my systemsn, but only when running DK2 & Leapmotion VR apps together (from the Leap store), using just DK2 or just Leapmotion seemed fine for these demos/games!
Speculation points towards my Nividia 660Ti not being good enough these days, alexcolgan (from Leapmotion) was very willing/helpful in the Oculus forum, here is something he said regarding GFX cards…
Quote" Post by alexcolgan » Thu Aug 27, 2015 1:23 am
We encountered similar problems with the 660 Ti, and the Oculus recommended requirements call for GTX 970 or higher. Your log indicates a TDR error, which is essentially what happens when your GPU simply gets overwhelmed and stops operating."
I have since then started to use my other computer system that has 2X1 Nvidia 660Tis and a 3GB Titan and most` crashes have been resolved, however I do still have a few problems…
My main frustration at the moment is with the Leapmotion side in Unity! Hands jumping in/out of view, multiple hands on screen when i am only using one hand over the LM sensor, virtual hands turning in to a fist while my real hand has all fingers open, hands being detected or shown upside down!, grab not working!, fingers in poses that my hand is not making…all in all it has been a VERY frustrating experience…and yet all the devs/youtube videos seem to have nice stable arms/hands and fingers with no grab issues!
I have done callibration SO many times (via monitor and using a mirror) and only score between 80-87 points MAX while I read that ppl are getting around 110-117 points!
With all these callibration issues (Tried from here: Downloads - Ultraleap ) and the other things I have mentioned, what was meant to be a hobby during the little down time I do get has been ruined by having to browes countless documents/ forum threads/youtube videos in the search for stabillity before I can even begin to have some fun with all this stuff 
Oh, and regarding the crash on or when exiting PLAY…
Thanks to this (http://forum.unity3d.com/threads/unity-5-1-vr-getting-started.332316/page-3#post-2255167) thread I now know about the sig file part that was indeed quite vague to say the least!
Found here: https://developer.oculus.com/osig/
