I could test this out on my Garmin 2450 unit, but can you, or someone else, provide me with test routes (US and EU, but EU would be most helpful) that would validate that all the files (JVC/SID/G2S) are working properly?

I would then list the files I have collected from various places after verifying them working.

Here are the results of my testing so far, only the IMG (map) and JCV (Junction View) files so far.

Unit: 2450

Garmin Europe Test Route coordinates
Start: N 53.47679 / W002.37047
End: N 53.47780 / W002.26610

Garmin America Test Route coordinates
Start: N 40.64078 / W074.20522
End: N 40.92326 / W072.71857

Testing JCV
Set 'simulation mode': Tools -> Settings -> System, first option, switch it to ON, defaults to OFF on each restart.
Create the starting/ending locations using the coordinates by going Where? -> Coordinates (may be different name as I don't use English menus).
Once you have the points saved in the unit. Go to Where? -> Favorites -> Coordinates.. -> pick the start location.
Then, click the upper left corner to open up a little menu and choose "set location". Finally, go back to the locations list and pick the stop location and hit GO!.

When simulating a route, you don't have to wait until a JCV screen is shown, you can click the Lane Assist bubble (upper left corner) and scroll through all the intersection data. When a JCV is available for a particular intersection/lane change, it will be shown in place of the regular map.

On the EU test route, for map 2012.40 using D2504070A.JCV (full screen, all maps), there are 3 JCVs, at lane change: 4, 5, 6. Ensure that you are going over the M602 to reach your destination.

On the NA test route, for map 2012.40 using D3060050A.JCV (full screen, all maps), there are 10 JCVs, at lane changes: 1, 3, 9, 10, 11, 13, 14, 15, 16, 17.

File location:
On the Unit (.System\JCV), from my testing, the JCV file must be in original name, changing the name cause the JCV, and possibly other JCVs, to not be recognized. Different map version JCVs can coexist in that directory.
On the SD (SD:\Garmin\JCV), from my testing, the JCV can be any name, but up to a certain length. If the name is too long then it will not be recognized. The following file was not recognized: Europe NT All 2012.40.JCV. There can be multiple JCVs in that folder for multiple maps. If there are different size JCVs for the same map then the biggest one is used. Ex: I had the D3191040A.JCV (portrait, all maps) and D3060050A.JCV (full screen, all maps), the D306.. was used by the unit.

Testing IMG (maps):
This is rather straight forward. The unit can accomodate a very limited number of maps. But the SD card can contain, it seems, and unlimited number of maps in the SD:\Map folder. The maps on the SD can be renamed to anything, I have not observed a name length limitation as with the JCV files. If two different version maps are present for particular region, the latest map is used. Based on the map used/selected, it's corresponding JCV will be used if available.

General rule for all files, got it from a post on one of the forums on this subject:
"You can put in SD only the map+3D+JCV file, if you want to update ASR+G2S+DB file you just have to ad this files in
existing folder in hidden .System folder, you will have 2 G2S files in G2S folder, 2 DB files in SQlite folder and all
ASR + SRX files in ASR folder, just remember that you don't have to rename nothing just use files like they are(all
this is tested by me in my37XX)"

Other notes:
I use the root SD to hold files temporarily that I don't want the unit to access, this saves time by not having to constantly delete and save files on the SD, which takes forever to transfer, a limitation of the unit itself, not my USB connection.

Having said that, anyone know why the portrait, all maps D3048050A.JCV is not working for my EU 2012.40 map?

Those work D3084040A.JCV, D2504070A.JCV (both full screen) but I prefer the portrait version.

Could my EU map be Unicode/Non Unicode and the JCV files distinguish between the two?