I'm thinking that clumping is more likely with flour. Am I wrong?
It all depends on the technique. If your cornstarch slurry was lumpy when you added it to the liquid, then Yes. You will end up with lumpy bits in your gravy. For the corn starch, do what Dazi said. It is imperative that you get all the lumps out of your slurry before adding it to your stock base.
Flour -- again, depends on your technique. If you add a flour slurry to your stock base, then it's real easy for lumping to occur. I've never found success with this method.
Are you totally opposed to the gluten-free rice flour? I posted upthread that I prefer a flour roux based gravy but it is a must that you wisk/stir constantly to remove the clumps while you slowly add the liquid a bit at a time . . . stir, add liquid, stir until smooth, add liquid, stir until smooth, add more liquid, etc. . . . It might take as much as 10-15 minutes of constant stirring to incorporate all of your pre-made stock. Plus this type of gravy stores well and is easy to reheat.
I usually use the corn starch method for small amounts of last-minute gravy . . . more like a thickend sauce type of thing.
Making a rich, thick, flavorful gravy really isn't as simple as it sounds . . . try the rice-flour roux. It sounds like you have plenty of stock, so you're good to go in that area.
At the very least, you have back-up gravy boxes to serve.