First - you cannot calculate MPM based on the longest routing you can find - you need the official one published on EF or KVS or elsewhere.

Second, what you are proposing *should* be possible: earlier in the thread, someone routed YFC-GRU-JNB on the outbound and JNB-NBO-FRA-YFC (I am skipping some parts, but gives you an idea). The return was a published routing on LH. Search for that post, wasn't too long ago, should give you some ideas.