Well I tried that newer file (I think you should update this package). It helped with identities. All tables are utf8_general_ci.
Now, when I do import on my local computer (php55 built-in webserver), it works just fine. All posts are imported in proper way with all characters.
On remote server however (php53) it still removes nonascii characters. I switched that to php54 - small change as it imported text with all characters but instead all the letters that was cutout earlier are changed to question marks '?'.
It looks like a problem with conversion or lack of charset setting on tables we import into. I tested XenForo_Importer_Abstract::_convertToUtf8 how it works on both environments and both results are the same. So more likely it's Db thing.
I could try moving local XF db to server, as it has properly imported content, but it means hacking settings in db to make it work on new domain, settings etc...
Beside, your importer doesn't import forum/categories permisions from FluxBB :/ Dunno if it's bug or it's intended.