1. Is there a possibility to save a tactical battle before it ends (ex.: after 5 minutes)?
No
2. Is there a possibility to begin again a campaign that we already have begun (ex. : 1 or 2 turns) without creating a new profile ?
Yes, Terminate (red button ) in global statistics
3. In the operational phase, I don't understand why we can't move two units to the same square.
Yes. One platoon = one square
For example, in the River Mzha campaign (Russian side), we should be able to put two or more companies on one of those 1 km2 square. Why this limitation ?
Lack performance on modern computers.
4. In the first turns of that River Mzha campaign (Russian side), the weak infantry companies encounter the motorized German units wich are a lot more numerous. As far as we can not concentrate our troops at the operational phase, does it means that we have to refuse combat and retreat until we get sufficient troops to beat the opponents ?
Why can not we?
In the battle involving platoons with the neighboring cells.
Put to the neighbor cell AT guns and other infantry platoon and during concentrate forces in tactical battle (in one square).
5. How do we ask an infantry to fire to a certain spot AND stay stealthy ?
It's not realistic
If you shoot you will be spotted soon.
Try to use hold fire command and mark prefferable targets.
How do we ask it to walk to a certain spot AND fire to a certain spot ?
Attack order and marks "spot" as prefferable target ([T]).