Just a question, in the Russian version (which, by the way, is planned), will these "ghouls" still be called that, or will they come up with something better?

Ghoul is a perfectly normal name ( http://ru.wikipedia.org/wiki/%D0%93%D1%83%D0%BB%D1%8C_%28%D1%84%D0%BE%D0%BB%D1%8C%D0%BA%D0%BB%D0%BE%D1%80%29 ). But nothing can be said about the Russian version yet; the localization team hasn't even been named.

The spider girl doesn't evoke much sympathy; a dried fish would have been better.

In my opinion, it's an original unit. In any case, the developers want to introduce something new; in the fifth installment, the Necropolis bestiary, excluding one unit, copies the one from the third game.

And what about the plot? Will it really take place before the events in "Heroes 5"?

Absolutely.

And what is the starting date of the events?

Approximately 400 years before the events in Heroes 5. The exact year has not yet been announced. In the fifth game, the events take place in 969.

And when did the Necromancers have the state of the Silver Cities? Is this not a mistake?

It's not a mistake. The events of the game take place before the Necromancers were exiled from the Silver Cities. For some time, they did indeed share them with the mages.