Moe, i've used it a few time and one time had a issue with it not drying in the middle, here is what I came up with.
1. I have yet to be able to reuse a can after it was started so if you did this it may be the issue. What seems to happen is that the "stuff" from a already used can never seems to dry, not sure if it is related to it not sealing right after being used.
2. It is a good idea to do it in layers, letting each layer dry first. This also makes the 1st issue a bit tough.
3. I have found it easier to make the rough structure with styrofoam (siliconed in place), and then fill in the gaps with great stuff, this takes less great stuff and in most cases only one layer.
For your issue I would drill holes in it to allow air to get the middle though it may take a long time to dry now. I actually had to remove half of mine and get a new can and start over when I had the issue. If you plan on having a water feature you may want to go this way, as some of it may never dry. I was able to tell by sticking a long nail in it and it came out with sitcky uncured great stuff on it.
As for it being unsafe I rememeber a number of people posting that as long as it is 100% dry it is 100% non-toxic. I have it in 3 of my tanks, and have had NO issues at all, 1 of which that has been up and running for over a year. I also have stryfoam backgrounds covered with weldbond\cocofiber in 5 tanks with no issues, though this method takes a long time to dry.
Hope that helps, let me know if you need anymore info...