So, finally I managed to find the solution. Button Remapper didn't work for me as it was possible only to modify 3 buttons (although rooted phone) and none of those was keyboard one.
I have switched the behaviour Y-Z and 6 and _ are at proper places now, i.e. "6" in a row with other numbers and "_" on the left bottom, next to shift. So full QWERTY now from QWERTZ layout on Motorola Pro+
How to:
1. Root your phone - solution found here:
http://ivehadmine.blogspot.com/2012/05/root-motorola-pro-plus.html worked quick and with perfect result
2. In order to view and change system files I used Root Browser Lite (you need to give all superuser permissions)
3. Navigate to (use Root Browser) "system/usr/keylayout" and you'll see several files ending with"_keypad.kl". You will need to find out which one is responsible for your layout (e.g. mine was "surf_keypad.kl"). I modified each file one by one and after restarting device was verifying if change in respective file caused any change in QWERTZ layout.
Before any modifications backup original files (e.g.copy to other destination but not in system folders)
3. Modification in .kl file is quick and simple - using text editor switch Y and Z positions, so now key 21 is assigned to Z and key 44 to Y. Save file and restart device (Quick boot app makes things quicker)
4. If you do it correctly, after restart you will have "semi" QWERTY layout. It means that pressing physical Z you will get Y, however alt+Z will result in "_" not "6". Same story with Y. To fix that you'll need to modify .bin file.
5. Go to "system/usr/keychars" and again you'll see several files with ".kcm.bin" extension. You have to find the one with same name as .kl file which you modified in step #3. In my case it was "surf_keypad.kcm.bin"
6. I modified ".kcm.bin" file on PC using XVI32 Hex editor (can be found here:
http://www.chmaas.handshake.de/delphi/freeware/xvi32/xvi32.htm#download). After opening file modify following strings:
"Z 6 z Z 6 6 6" to "Z _ z Z _ _ 6" and "Y _ y Y _ _ 7" to "Y 6 y Y 6 6 7"
Save file and replace original system one.
Remember to backup/copy original file! Restart device,
After reboot you should get QWERTY layout with proper 6 and _ placement :fingers-crossed: