Re: substrate injestion...
I've never kept gecko's but when my wife feeds her adult bearded dragon she moves him into a tub with nothing else in it. This way no crickets go uneaten, and no substrate taken in. I don't know if it would be difficult to do this for gecko's or not, the dragon only gets crickets every few weeks.
Re: substrate injestion...
I use coconut fiber and both piper and wilson have ingested it a few times during "hunting" by accident and never had a problem. I use a bowl to put the crickets in, they usually stay in there and eat them in there as well. Hope this helps...