The 8kt thing is no problem. Just tell them you can do it and they won't care what you actually do (you will be doing the trip in 2 parts
taking 2 days, so no issue with speed).
I don't think there is much leeway on the handlers and pilot. Why can't you fit 6 people on the boat for motoring through a ditch? The pilot leaves at night, so you would need to sleep 5 on board - very tight, but doable for a single
Your alternative is to ship it overland - something more easily done with a boat your side, but I don't know any details for doing it in Panama
. I know there are shippers in Guatemala
that can take boats across, so maybe you go there?