All the headdresses that have just a front piece and don't cover the top&back of the head would be a venets/koruna etc (wreath/crown) — a headband, a young woman's festive and wedding headgear. Those can also get ridiculously high and wide, but don't provide full coverage. They could also be paired with a braid decoration.

Now, the mixing up starts as early as late XVIII for aristocracy and XIX for everyone else. Thanks to Peter I's "embrace Europe, reject tradition" policy the upper class was pretty much entirely cut from Russian traditional culture. When the "slavic" style came back in trend (several times during wars) it ignored all original symbolism and was heavily changed to fit international beauty standards of the time. By the time of Nicholas II (and yet another wave of slavomania pre-revolution) every court headdress we see is clearly a headband/diadem in its structure but every description states "kokoshnik".

Important note: in 1830s court dress regulations there is still a clear difference between kokoshnik and a headband for married and unmarried ladies accordingly. However, every history text elects to call every headpiece a "modernized kokoshnik" In the latest years the "kokoshnik" (BUT ACTUALLY A VENETS/HEADBAND) became an insanely popular accessory in Russia and is made in tons of materials but in a very limited number of shapes. The results vary from beautiful to silly to atrocious (I'll spare you from looking at patriotic take on the mater). I'd love to wear one, but, to quote marquis de Custine, "it flatters beautiful faces and makes ugly ones even more so"