I have a 2520 with a 200CX, 62D, independent lift, and a diverter valve. I know that you are already aware of some of what I'm about to say, but I'll say it all for completeness.
The independent lift uncouples the MMM lift from the 3PH and allows you to use the curl circuit of the SCV to hydraulically raise and lower the MMM. The independent lift has it's own mechanical height adjustment that limits the downward travel of the lift mechanism. See the attached photo that shows what this adjuster control looks like. In addition to controlling the mowing height, it also is used to lock the deck, or lift mechanism, in the fully up position.
Normally, when switching to use the FEL, you would lock the deck all the way up and then disconnect the hydraulic lines to the lift cylinder and connect the curl circuit for the FEL. The diverter valve allows you to have both the FEL and independent lift connected at all times, and then select between the bucket curl or the lift cylinder using a push/pull knob to switch the diverter valve from the operator seat.
Yes, the lift cylinder will leak down after you raise it hydraulically, but it's not really a problem. The hydraulically controlled part of the lift mechanism was intended to allow you to temporarily lift the deck to avoid obstacles, not as a method of adjusting the mowing height or parking the deck in the up position. The height adjuster of the independent lift allows for both mowing height control and the ability to lock the lift mechanism in the raised position, all from the tractor seat.