On your question about how to collect turtle poo - lots of turtles poo right after eating. You might try feeding her in a separate bowl, then waiting around until she poos, and collect the poo with a little fish net or a turkey baster (you don't want to use this particular turkey baster any more for basting turkeys

). You can store it in a little plastic storage container or a plastic baggie.
By the way, if it's very difficult to get her to a good herp vet, you could ask your regular dog/cat vet to check the sample. I don't want to discourage your also seeing a herp vet but in a pinch, any good vet should be able to check a stool sample. Beyond this, you'll need a herp vet for any sort of turtle health problem.
One more thing - I prefer making good quality pellets the main component of a turtle's diet. It's very difficult to recreate just the right balance of protein, carbohydrate, fat, vitamins and minerals that a turtle needs with a home made diet. Reputable pellet makers have done research and know precisely the balance of nuitriants in their pellets. I also believe even good quality pellets need to be supplemented with fresh veggies and other foods. I would see your gelatin based food as a supplement to good pellets rather than the main food for your turtle.