[rudrood]

@Milhouse : Is the 23.976 file working oke for you with the legacy driver?

Yes the 23.976 file is switching correctly - the log includes the playback of both 23.976 and 24.0 fps videos on my Revo 3700/ION2 into Onkyo AVR/Philips TV: http://ix.io/1aAw

Grepping the log for "ADJUST":

text:
00:59:28.688 T:139893793081536 NOTICE: Display resolution ADJUST : HDMI-0: 1920x1080 @ 23.98Hz (19) (weight: -0.000)

00:59:30.242 T:139893793081536 NOTICE: Display resolution ADJUST : HDMI-0: 1920x1080 @ 23.98Hz (19) (weight: -0.000)

00:59:56.613 T:139893793081536 NOTICE: Display resolution ADJUST : HDMI-0: 1920x1080 @ 24.00Hz (18) (weight: -0.000)

00:59:58.126 T:139893793081536 NOTICE: Display resolution ADJUST : HDMI-0: 1920x1080 @ 24.00Hz (18) (weight: -0.000)

(Yeah, two ADJUST messages for every video is an Nvidia thing)

Not sure why the "weight" is -0.000 - I tested it several times and the weight is usually a large number:

text:
LibreELEC:~ # tail -Fn+0 kodi.log|grep ADJUST

00:53:43.090 T:140689497344192 NOTICE: Display resolution ADJUST : HDMI-0: 1920x1080 @ 23.98Hz (19) (weight: 474942504742052212056560953720832.000)

00:53:44.677 T:140689497344192 NOTICE: Display resolution ADJUST : HDMI-0: 1920x1080 @ 23.98Hz (19) (weight: 475001306893918267619618662121472.000)

So that's a little odd.

IF so: could it have something to do with how I load the legacy driver http://paste.ubuntu.com/p/dGvBcqwd3N/?

I don't think that has anything to do with it.  

 Thanks for testing. Will have to live without fps switching on this system.
Don't want to whine, but I think the whitelist function seems to criple more than it brings. Before it was introduced framerate switching was working 100% on this system.
In my opinion it replaces a perfect, easy auto switching system with a method most "standard" users will have some problems to uderstand and make work.