1. Depending on how much sauce you have, you may need to add more milk/cornstarch mixture. Just add 1/2 milk with 1 tablespoon first and see how it thickens up. If needed, repeat the process.
2. Originally I posted this recipe with the lemons added to the crockpot. It happens that the sauce can get a little bitter from the lemons. I guess it depends on the lemons. I suggest to skip the lemon and adjust the tanginess of the gravy at the end with some lemon juice.