Building code compliant prefabricated rocket stove, safety tested

We just purchased a Liberator stove a few weeks ago, to replace the propane we installed when we bought our house. The house originally had a woodstove, but we were gone 14 hours a day at work and wanted something with a thermostat. Now, I'm home all the time and with rising fuel costs, it just makes sense to go back to wood heat. My husband ran across the Liberator and loved the design so here it is!

It is currently sitting on the pallet it shipped with in our garage so we could learn how to use it before we HAD to use it. The pellet hopper tube does have to sit at exactly 19 1/4 inches. We found that out pretty quick.  We have been trying different lengths of stove pipe, too, and this is where my question comes in. We have yet to get this thing really roaring-that is, making the sound rocket heaters are known for. The smoke coming out is still smoky and my understanding is that it's not running correctly if this is happening??

According to the instruction, we can't use more than 12 feet of horizontal pipe but it isn't clear why. All the videos we've seen on RMH seem to have a lot more than 12 horizontal feet in their exhaust. With this restriction, our mass (which will get built next summer because we have to crawl under the house and reinforce the floor) is only going to be about 5 feet long, allowing for the pipe to do a 180 as it comes back to the vertical chimney pipe. Is that size mass enough to hold heat in a 1200 sq. ft. house?

Other comments on this thread mentioned having the stove closer to walls than building codes allow but the instruction give dimensions for installation of 12 inches from the side of the heater to a wall.  We are going to use a piece of corrugated metal as a heat shield, regardless, but I still think our stove will sit a little further out. The original wood stove in our house sat on a faux (super ugly) brick hearth that ran up the wall. We ripped all that out when we remodeled but my husband built a granite pad for the propane stove to sit on. I'm not sure how high the temps get under the Liberator. Is it a good idea to sit it higher? I should mention that our house is 102 years old with nice flammable cedar shiplap behind the drywall.