can a cast turret like those of the russian MBT T-90 fire its main gun?

Answer 1

Of course it can
The turret is cast, but later is machined- at the turret ring, cannon mount and hatches. The gun can move and fire just as well as in the welded turrets
It is simply a question of design, nothing more. Of course in the case of welded turrets it is easier to introduce novelties- like a change in armour quality or thickness, etc. In the case of a cast you have to make a new mould and then research it (new angles of armour slope) Also it is more vulnerable to cast problems- impurities and such.