I agree that the problem with a concon being a runaway is not going to happen. Because I don’t see the concon happening at all, and for much the same reason you’ve discussed why it won’t be a runaway convention: you need 3/4 of the states to ratify. Which means that 1/4 plus one can scuttle anythIng that comes out. Here are 15 states that would be likely to do exactly that;
HI, CT, MA, NY, CA, RI, VT, WA, OR, MN, NJ, ME, NM, DE, MD.