If you've done flannel without batting before, go for it. I personally find it to be even hotter than regular cotton backing with batting, but it's your quilt.
The way flannel is made is by trimming down cotton fabric, making it a short & dense. I love the softness of it, but I overheat with it quite quickly. The other thing I'm not sure about -- so maybe try a sample sandwich to see -- is how much you will feel the stabilizer through the flannel if you don't us a batt.
I've tried ironing on a sheet before & would still recommend the paper grocery bags. I know it sounds insane, but honestly, it makes a big difference. The sheet can bunch up -- even if you've taped it to the table, if the vinyl starts getting a bit too hot or some of your fusible leaks out, you will end up with the t-shirt stuck to the bed sheet (ask me how I know). At a minimum, I'd iron some parchment paper to the sheet if that's still the way you want to go.