AFAIK, in this context there are two types of fare:
- Fares where you have to use a specific routing, as specified in the Routing Rules (eg: LAX-AA/IB-SJU/NYC/MIA/BOS/CHI/WAS-IB-MAD/BCN-IB-MAD/BCN-BA/IB-LON* in your first example above)
- Fares where you use any routing that is within the MPM (and subject to certain other rules). This is what "MPM TRVL PERMITTED" is referring to in your second example, and the 8848 is the MPM.

So, in your case, you can choose your desired routing for NYC-SHA, and then use the KVS TPM calculator (under 'Reference') to find the mileage for that route, and check it is less than the MPM. You also need to check the fare rules in case there are any specific restrictions.

See Q44 in the FAQ and http://www.kvstool.com/FAQ.php?Source=HOME#MPM for further explanations.