Nevermind, I'm pretty sure I figured it out, I think I was very overthinking it!
Looking at the solutions for chapter 4, I solved the top part of the left page a bit differently than you did, and I don't see why my solution is wrong, here's how I did it (rot13 encrypted)
Gur gjb oyhr wryyvrf ba gur gbc evtug bs cntr 42 (jvgu gur lryybj fgnva orgjrra gurz), vg jnf pyrne gb zr gung gur gbc wryyl pbhyqa'g tb fbhgu, ohg vg'f abg pyrne gb zr gung gur obggbz bs gur gjb pbhyqa'g tb hc. Zl fbyhgvba unq gur obggbz evtug wryyl tb hc naq nebhaq pybpxjvfr, naq gur gbc yrsg wryyl jrag yrsg gjb fcnprf, qbja gjb fcnprf, gura yrsg naq nebhaq gur erq fgnva. Guvf yrnirf rabhtu ebbz sbe gur lryybj wryyl gb tb evtug gjb fcnprf, qbja bar, gura evtug (whzcvat gur oyhr) hagvy vg ehaf vagb gur obggbz evtug oyhr wryyl. Gura vg tbrf qbja (whzcvat bire gung oyhr), evtug, gura qbja gb gur lryybj qbg naq zretrf jvgu erq gb cnff benatr gb gur bgure cntr.