Thanks for the framing layout. It'll come in handy when I add an entry assist handle. My middle support on the ladder is at 43 1/2" O.C. from the bottom trim. If they missed a backing location, it's on them. I will say I've climbed up and down the ladder more than a couple dozen times and everything looks good, so far. I hung from the top bike rack mounting plate (angle aluminum) and nothing budged, so I'm fairly confident the ladder can hold the rack and bikes' weight (I'm 205#). I might investigate the framing numbers a little more in depth when it's daylight, although it's too late for the existing ladder.
The bike rack wasn't my idea. It was inspired from this forum and Google pics. I didn't like the chosen hardware for the examples I came across, so I made my own design. One way or another, our bikes are going with us. I hope my amateur engineering will suffice, so I don't need to revert to the hitch mount.