I think that you'd have issues with the pressure hitting the tank both from the water weight and the soil.
What I would do is sell the 100g tank and use the money to buy a much cheaper stock tank, you definitely wouldn't be losing money there. Plus you have the added peace of mind that the stock tank CAN safely be buried.
As for your backyard- is it fenced in or contained? If not, you'll at least need something around the turtle area. Turtles are roamers, and they don't have the sense to return home to their pond at night. So if yours isn't fenced in it will simply take off.
