I feel that what is being asked here is why doesn't the FMA change to VNAV SPD when speed intervene is pressed?  During a normal VNAV descent, the FMA will be VNAV PTH. However, when speed is intervened the FMA should become VNAV SPD.  The logic to this is VNAV is no longer honoring the path, but rather speed.

Also, for the first post:
Bartalos wrote on Dec 22^nd, 2018 at 11:59pm:

Hello, can anyone help how to engage VNAV SPD mode instead of VNAV PTH which forbids speed restrictions ?

If the FMC is programmed correctly then VNAV PTH shall honor speeds and altitudes.  I haven't played around with the sim too much to check it but the times I have, it has worked well.  If winds are a factor especially a strong tail wind, then VNAV PTH may not be too steep to maintain and honor speed.  In this case you will receive the scratch pad message "DRAG REQUIRED". 

Hopefully this clears some things up for all parties.