I now understand your confusion to its full extent. Well, I'll try to explain it the way I have understood it (perhaps in a kindergarden manner

), but I'll try to keep it comprehensible as well.
Hypoxanthine and guanine in general have two fates, either to be metabolized into uric acid and be excreted in the urine (metabolic pathway No 1) or to undergo metabolism through PRPP & HGPRT and produce IMP and GMP respectively (metabolic pathway No 2). Ok so far?
Once 6MP is activated by HGPRTase, it blocks PRPP. This blockade of PRPP will shift the metabolism of ENDOGENOUS hypoxanthine & guanine towards the production of uric acid (thus precipitating gout). This shift of purine metabolism to uric acid production will also affect 6MP itself. So, if you also block metabolic pathway No 1 with allopurinol, you force purines not to be metabolized in any way. The fact that 6MP is deactivated by XO only adds to the problem, because allopurinol also inhibits this surplus inhibitory alternative, thus rendering high circulating levels of the drug. The net effect will be a compilation of endogenous purines and 6MP.
Am I making a mistake in this rationale?