This is an unusual situation. From reading the thread it's been reformatted with RMPrepUSB a couple of times, and now has had NV cleared and chkdisk run w/o error, right?

If you want to save yourself some time in future assuming you're now stick with using a nuvi stunt-double to download future LM map updates, you could simply use only a microSD for that and leave the map and vcj on the card for it to be used in the nuvi. It seems you have no problem with smaller files and firmware being updated directly on the unit with WedUpdater, so use MapUpdater for the map/jcv to the card with only Garmin/GarminDevice.xml on it. The folder .System will be created with gmapprom.img and unl/gma files in it and sub-folder JCV with the *.jcv file. Move the entire JCV folder and the map img and associated gma/unl files into the Garmin folder on the card renaming gmapprom to gmapsupp and delete the .xml file. Remove the earlier version map from the unit if present and insert the card which will work perfectly. You can also make an sd root folder named Map and use <anyname>.* instead, [eg NTU_EU_2015.20.img]. JCV folder must stay in sd:/Garmin though.

If you don't find a fix for the internal memory problem the above will at least save you shuffling files around on the unit.