As far as I know, the Nikon Coolscan V ED (i.e. LS-50) does not have CFL but DELs which have high longevity and should keep bright along time. It shouldn't be necessary to replace them.
Moreover, the scanner is not supported anymore by Nikon since 2013 which means that parts are impossible to find. Many repair shops refuse to repair Coolscan scanners because they can't find the parts anymore. Consequently, I highly doubt that they replaced the lamp.
More likely, the service people simply cleaned the mirror.
The dust issue explains all the problems you mentionned here.
You can clean the mirror yourself and you will save money.
This is not easy but you can do it if you carefully follow this tutorial (the more detailed I found with clear photos):
Note that this mirror cleaning tutorial will work with almost all Coolscan film scanners because they are globally all built the same way.
I personally have the LS-50 scanner and this tutorial explains how to proceed on the LS-5000.
Inside, everything is quite identical and at the same place (of course, the electronic components and the sensor are different).
I had the same issue as you. I cleaned the mirror myself and had the initial picture quality restored.
ONE VERY IMPORTANT THING
Don't panic if after reassembling everything/power and running the Nikon Coolscan software, Windows raises a scanner hardware error.
This is because you installed the mirror upside so the scanner can not focus.
Both side of the mirror are composed of a reflecting surface which one side covered by a thick glass.
The reflecting surface must be installed directly in front of the lens (the thick glass is backward).
If you get the hardware error, just reverse the mirror and everything will be fine :-)
Finally, you will probably have to repeat the cleaning operation several years later because the mirror is direclty exposed to the outside dust so you'd better learn how to do it yourself.
Moreover, do not forget to cover the scanner when you do not use it to limit the dust exposure.