So, if you'd still rather make your rice-flour gruel work instead of using whole or coarsely ground rice, I have a couple suggestions that may help.
First, you note this blobbing up happens at a certain ratio of rice flour to water. Use less rice flour (or more water) to keep the mix fluid. The rice will thicken as its being cooked, so have the extra water already there so it doesn't thicken too much. If it's still too liquid after it's cooked, you can cook it thicker (via evaporation) until it's the right texture, which is slow but easier than trying to thin down the blob once formed. With time, you can figure out the best ratio and cooking time for what you want.
Second, you mention part of the problem is that the rice flour doesn't cook evenly when blobbed up - you may be able to pre-cook the rice flour, say by lightly toasting in a pan. This may also impart some color and a bit of flavor, as toasting usually does, though if you don't want it this may be minimized by using low and gentle heat. The idea is, once the flour has been toasted, it will not taste raw even if some parts of the gruel don't heat up as much.
Third, you might try a different order of preparation - maybe boiling the water first, then slowly adding the rice flour in. That would mean the water the flour comes in contact with is hot enough it should get the flour to cook even if it blobs up. Also, you can stop adding flour if it starts to pull away at the edges, and add a bit more water to smooth it down.
Finally, even if it turns into a blob, you can add water in to get it back to a manageable texture (slowly, transitioning slowly will usually prevent lumps). Go slowly and keep stirring, and it should loosen up from a thick blob to a thick paste, and then into a thinner paste, then into the more liquid stages. Lumps happen when mixing very thick or dry to very liquid, going through the paste stages usually prevent them.
And lastly, if you want the firmer texture which happens when it pulls together, you can pull the pan off the heat once it's a dough, split up into smaller knobs, and cook like that - maybe boiling like dumplings, pan-toasting, baking, steaming, whatever. These methods will let it cook even when solid. This would be very similar to making mochi, as Jude mentioned, so looking at those recipes may help.