Hello!
This first issue sounds like the same thing I have ran into recently with the levers in my test game. I think I may have mistakenly introduced the bug in a recent release. I meant to look at this sooner but kept getting sidetracked - I will try to take a look today!
In regards to the difference between the two types, "Once" plays through the animation frames once, then returns back to the start frame. "Clamp" plays through the frames then stops at the last frame and stays on it rather than returning to the start frame. At least that's the way they are supposed to work. :)
Would you mind e-mailing me the chest objects and script you are using to animated the lid? Being two separate objects that have their own processing logic, I'm not sure it's guaranteed that their animations will be triggered at exactly the same time, but I'll mess around with it some and see if I can tell why it's not consistent. When it does occur, how out of sync are the two pieces, time-wise?