At that time of year there seem to be no Y seats available from GUM-ROR, even as an elite. There are F seats available on many days in December and January, but nothing in the back of the plane, even for a CO Platinum. GUM-PNI doesn't even have anything up front.

Also, PNI-YAP-ROR is not a valid reward routing for an open jaw, which is what I think you're trying to do there. The "open" segment has to be the shortest of the three sides of the triangle, and in this case PNI-ROR is actually the longest.

I have redeemed for F on the GUM-ROR route before (in August), but I'm guessing that demand for paid Y the flights in December/January will be pretty high because of the holidays. I'd try to swing the miles to get the F seats and go from there. If you fly up front, try for the 767s on CO between NRT and GUM. They are the BF seats, though not the service. But they're way nicer than the seats on the 738s.

As for tools, the only option is CO Micronesia, and the flight search on co.com is pretty much all you've got for searching that. The monthly calendar is pretty useful.

Also, if you're planning on diving in ROR (which I hope you are if you're going), I recommend a liveaboard. We did shore-based and were rather annoyed by the long rides out and back to the dive sites each day.