Hey Nox, good stuff. I was gonna do one of these myself, but you beat me to it.
Three points:
#1: You have to beat 1d4@CR wandering monster encounters, and 3@(CR+3) normal encounters. Monsterous Crab is CR 7 = good luck. NPC optimized can have some really bad stuff, just low gear. There's no listing info about how big your party can be, neither about how long you take to do it. So big party, multiple days. You are nova-ing, monsters are bringing in re-enforcements. Alot of DM fiat possible here.
#2: Sage ruled in with (i'm semi-quoting here) Advancement follows normal rules. OK. I'll see if I can link it.
#3: My example stands. Wizard 8 / PrC 2 is a 10hd creature. Rebuild 2 levels after successful quest. Woops, you PrC isn't functional now for some reason. What are you? A Wizard 6 / "something" 2 / non-functioning class 2. In other words, as simply as possible, the worst case scenario is Wiz 6 / Commoner 4. So you happen to have had Practised Spellcaster feat, not doing much previously. Your CL is 10. You immediately requalify for the PrC (perhaps), now you are a Wiz 6 / PrC 2 (very weirdly qualified for) / 2hd waiting to be rebuilt. So you take the PrC, to be finally, Wiz 6 / PrC 4.
Conclusion: What we are doing here, does not violate the book's paragraph 3 or what the Sage ruled. It does use paragraph 4, and the following 2 pages. More clarifying text is in Racial rebuilding right next to it. "Rebuilding racial levels is difficult" is what the text says. It doesn't say that about class rebuilding. Commoner.
Let's call this what it is. Overqualifying.
The rules also state what you get to keep, during the ~instantaneous transition. BAB skills feats etc ...
Some minimal amount of paper work, and attention to detail. At the very least, count the iffy levels as Commoner; if you still qualify, lucky you. Using a class to extremely complicatedly qualify for itself, is fun. You can do this over and over, not that anyone will, slowly but surely getting slightly more powerful each crazy quest and rebuild hash.