I'm sorry I can't help w/the traveling companion but I can tell you about our bus travel. We did the same thing years ago the first times we went to BZ but things I am sure are changed
. I "think" you can get bus from Cancun to Playa but not sure about your departure timing. We took a private vehicle to Playa cause we had a large group and the fare worked out fairly for us. Also we were traveling early in the AM and we did everything in one day. Someone had a bus schedule link on an earlier post. Use the "search" function above. And make sure to have correct $ for ticket ~ at bus staton no one had change and they only took pesos when we did it. The bus was very nice, air conditioned and tv but it was a lo-o-ong day of traveling with lots of speed bump "stops" and then 2-3 stops for Customs checks. We took the bus to Chetumal, had a van that met us there and took us across border and then to the airport in Corazol and then we flew over to AC. The first time we went we stayed overnight in Chetumal (and loved it), walked across the border and met our driver for ride to airport. Both were really no problem ~ now we just find it's easier to fly into BZ City and come across. But you've already got tickets.
IMHO I don't really think either of your tentative plans is really too much to do in your 14 days. Hopefully someone else will give you some pointers. I would get to the mainland if you can ~ one year we did Lamanai and Orange Walk area (we had a driver)and one year we stayed in Santa Elena (we had our own car) and did Tikal (which was awesome!)
Have a great trip!