[Imp]

In game terms perhaps placing smoke by arty is slightly more effective as its easier to judge where to place it than it would be in real life. Of course this is countered by the fact that many other means of producing smoke are not modeled.
As to duration yes wind is always the same strong winds are not modeled, but smoke was used & maintained very effectivly & of course smoke grenades dont last half as long as shells. Screening for several hours or even I think days did indeed happen using smoke generators.

As to the other things you mentioned most armies realised this & had doctrines along these lines
Smoke hampers defence far more than attack, planing of firelanes is therefore cruicial.
Be prepared to defend important locations by hand to hand combat.
Be prepared to counterattack once the smoke starts lifting.