Is there any scope to move your broody hen to a different nesting box where she won't be disturbed? That's a shame that two of the eggs got broken. I'm not sure about incubating as I've never done it, my broody hen has hatched all the chicks.
If you see movement that means the chick is alive so I would definitely take that as reassurance. If you have a broody hen that’s been sitting awhile and you trust her, you could put the eggs under her now? I put two 14 day eggs under my broody last time she wanted chicks and she hatched them...
If in doubt, leave it. You can mark that one with a pencil and re-candle at 17/18 days. If it is has stopped developing it should be more obvious then.