Heroes 5.5 WarGame Edition - Tactical Edition.

Yes, there is less of a penalty on the initiative scale, and mana is not consumed at all; I checked this with summoning – the test hero ran out of mana :D. Therefore, out of 12 summoning spells, I tested 11 (I didn't have enough mana for the phoenix); all the spells consumed mana except for the wall, the hive, and the crystal. I suspect that the fire wall also won't consume mana if cast in an empty space.

Added 2 minutes ago
In general, the summoning of the crystal and the wall was near the unicorn – perhaps resistance was triggered (this was not displayed in the log or visually on the battlefield, but mechanically, it is possible that resistance was calculated).

Added 3 minutes ago
I checked everything – the phoenix consumed mana; three casts are free and without any resistances.