Why would the US occupy Colon in 1885? At that time Nicaragua still had the inside track as the most attractive location for a Pacific canal, since to my knowledge that Nicaraguan volcano had not erupted yet.
Could Chile have unified the Andean countries the way Prussia did Germany starting with the Nitrate War in 1870 and annexing Bolivia and Peru, then on to Ecuador and Colombia? Then Central America by 1885? If Great Britain did not intervene, I'm pretty sure Chile could have pulled off the first part of it if she consolidated her power by extending a railroad or two from Santiago north to the Ecuador border before going any further. Once THAT Chile got say, as far as Nicaragua, let alone the Guatemala-Mexico border, the US might become worried enough to take military action if it became clear that the UK wouldn't.