how to induce underpinning? (Blitzkrieg)

I do not know.

No way. Reinforcement is a function of the type LoadReinforecement, it refers to the map script, and not to the actions of the player in the interface. Those. conditions are written (there are no computer units left in the map area or, on the contrary, it is occupied by the player and many others) and upon reaching them, reinforcements are loaded, either statically or with a command to do something, for example, move along the coordinates or into the map area.

There is a drop of infantry. And there is no more.
What you were given is what you fight.
So think before you do anything.
Pause for space.