as to caching plugin, you can use it, but need to tell it not to cache the wp page the forum appears on… this is a setting in w3tc but you have to do it for each caching type in question…

unless your site is just getting killed without w3tc (highly doubt it since it doesnt really help much), I would disable it completely at least while trying to figure out your local issue with posting…

manipulating forums (merging, add, creating new, etc) would have no impact… it doesnt affect anything that would have anything to do with post length… in fact, we dont care about post length at all – assuming of course it fits within the php limits as defined in php.ini file…  cant imagine settings though that would have issue at 650 words though you might check post max setting….