Alright, so this is my first batch where I came up with the recipe instead of using a recipe kit, and something seems to have gone wrong. Here are my brewing notes so you can see exactly what I did:

So, I brought 3 gallons of water up to a boil, then added all 13 pounds of malt extract, which actually almost filled up the 5 gallon kettle. After brewing, I added maybe 1 more gallon of water to the fermenter, then took a gravity reading. (It was a 6 gallon fermenter, but wasn't quite full and had a crap ton of sediment, so I'm guessing I had around 5 gallons) Here is where the first unexpected thing happened - my OG reading was 1.072, which seemed really low, especially since the calculator I used estimated 1.095.
I'm on a time crunch to have this ready for a homebrew beer fest on Feb 25, so I bottled it after 2 weeks in the fermentor. I assumed it would be done fermenting and did not even check to see if the airlock was still bubbling... Which obviously I would go back and change if I could. So we siphon as much beer as we can out of the fermentor, and end up with 3 gallons. Not exactly sure how, but there was so much sediment in the fermentor that it was hard to get clear beer out when we were near the bottom.
This is where I start to worry - we take a gravity reading and it is 1.040. Now I'm a little worried that there are still fermentable sugars in the beer, especially since I didn't check to see if it was still bubblin, and the calculator estimated FG at 1.027.
One more issue - I put in 3/4 cups of priming sugar, which is the amount for 5 gallons, and I only had 3.
So here is my question - should I be worried about my bottles exploding? Also, can you think of any explanations for why my OG was so low? It doesn't make any sense to me. And on the FG... Is there any way that fermentation could be done with it being that high? Thanks for any clarification you can give.