The MONTAGE will report on how much data you have loaded by going to [UTILITY] > touch "Contents" > "Data Utility" > then accessing the folder you wish to report on...
If by "free ram" you are referring to Waveform memory, touch the FOLDER labeled "Waveform"

The upper right corner of the screen will report how much memory is used out of the total 1.8GB available memory for the MONTAGE
Listed in the main portion of the screen are the Names of the Waveforms currently occupying the USER Ram area.

The various FOLDERS will show you what you have loaded in the USER area, when you enter them: ARPs (out of 256), LIBRARY (out of 8), LIVE SETs (out of 8 banks), MOTION SEQs (out of 256), PERFORMANCES (out of 640), SONGs (out of 128), and WAVEFORMs.

My second question: How is it possible to switch beetween the led's of part 1 - 8 / 9 - 16 and element /operator?

On the right front panel work you must learn to work with the [PERFORMANCE CONTROL] and the [PART CONTROL] buttons. Most times when you recall a Performance, and you are on the HOME screen, the [PERFORMANCE CONTROL] button is lit and the upper [COMMON] button is lit brightly.

[PERFORMANCE CONTROL] - designed to make it easy to control the KBD CTRL Parts available for simultaneous play:
When this button lights, you are initially set to "control" the current Performance.
The top row of eight buttons across, are PART SELECT 1-8 (dotted white line extends down around the left of the buttons).
The second row of eight buttons across are PART MUTE 1-8 (switch a light from bright to dim, you've muted the Part).
Row three across are MOTION SEQ SELECT 1-8.
The bottom row are ARP SELECT 1-8.

The upper [COMMON] button is on the same plane with the Part Select 1-8 button (this is no accident). When you directly *select* a Part 1-8 on the top row, while [PERFORMANCE CONTROL] is lit, you have entered that PART. If you are on the normal HOME screen you will now view the 8 PART Assign Knobs for the "selected" PART... instead of the 8 COMMON Assign Knobs.

The act of selecting a PART, by pressing a [PART SELECT] button 1-8, automatically flips the function of the Faders from Part Volumes, to the selected Part's 8 Element Levels (AWM2) or 8 Operator Levels (FM-X). Real-time access to the selected Part's Oscillators in this fashion can mean balancing the response of an instrument can be done without a bunch a menu moves (and screen changes)

[PART CONTROL] - designed to give you access to any of the 16 Parts of a Performance - as individual entities.
This button switches the front panel functions to those written down the right side of the button grid, as follows:

Notice all 16 Part buttons of the first two rows glow dimly when the upper [COMMON] is lit - when you select a PART, 1-16, that "selected" Part will glow brightly and the slot in the screen will be selected (highlighted).
Row three is MOTION SEQ ON/OFF
Row four is ARP ON/OFF

When [PART CONTROL] is lit, the Faders become Element/Oscillator Level controls (this is useful when setting the Faders to act as Level controls like when acting as pseudo-'drawbars'). You can STORE the Performance so that this type of Element/Oscillator Control is available when recalled - in such an instance the [PART CONTROL] button will flash.

Extra Credit:
Normally you are viewing PARTs 1-8 when you recall a Performance. Pressing [SHIFT] + [PERFORMANCE CONTROL] automatically flips the view from Parts 1-8 to Parts 9-16.
The button will flash

As mentioned, pressing [PART CONTROL] makes the first two rows of buttons 1-16.
Pressing [SHIFT] + [PART CONTROL] gives you access to the Element/Operators for the *selected* Part.

You can STORE the condition of these two very important buttons... You may notice on the B3 Organ Factory sounds that access to the individual Element Levels is the stored condition... making them pseudo-drawbars.
You can see these parameter settings within the MONTAGE system: From the HOME screen [SHIFT] + [HOME] this is the shortcut to the "Motion Control" > "Overview" screen.

In the upper right corner you can see the "Control Function" options. These determine what the Faders are doing: “Performance Control”, “Part Control”, and “Element/Operator”. Notice how changing these correspond with the front panel buttons.

Thank you very much, bad Mister, for your perfect supply. Your instructions helped me a lot and has been easily to understand.

Greetings Jost