So, Sony doesn´t only tweak the base ISO (which they are allowed to do) but they are compressing the ISO scale what I consider cheating.
It's still not cheating so long as whatever nominal ISO Sony designates for any given ISO tells you the relationship between expected output brightness and exposure. In other words, according to what you're saying Sony uses less analog gain and more digital gain for very high ISOs. This does not confer any advantage when it comes to apparent image quality at nominal high ISO. In fact, it usually confers a noise penalty.
Lets take an example where two companies use the same exact sensor in two different cameras. Here "analog gain" refers to DxOmark-measured ISO and correlates with sensor "sensitivity" while "digital gain" refers to "pushing" the file in the JPEG engine or supporting RAW converter.
In order to produce an image of "ISO 200 brightness" for a given light level, shutter speed, and aperture:
Camera A applies "ISO 200-equivalent" analog gain and no digital gain.
Camera B applies "ISO 100-equivalent" analog gain and 1 stop of digital gain.
For a typical current sensor where read noise declines with increasing ISO, this means that the resulting file from Camera A will have less shadow noise. Regardless of sensor technology, the RAW file from Camera A will have 1 stop less highlight headroom.
Now lets say that we are using the same two cameras to produce an image of "ISO 1600 brightness" for a given light level, shutter speed, and aperture:
Camera A applies "ISO 1600-equivalent" analog gain and no digital gain.
Camera B applies "ISO 100-equivalent" analog gain and 4 stops of digital gain.
In your terms, Camera B is the ultimate "cheater" here. In reality for a typical sensor, all that pushing will incur a noise penalty. The tradeoff is that by keeping the analog gain at its base value, 4 stops of extra highlight headroom is preserved in the RAW file.
The bottom line is that for a camera to produce a file of a certain desired brightness, dynamic range has to decrease under low light, shutter speed-limited circumstances. When a company alters the relationship between DxO-measured ISO and nominal ISO at a given nominal ISO value, all they are doing is choosing a tradeoff as to how to spend their available dynamic range (ie, on highlights or on shadows) for a given set of circumstances. There is no cheating.