What is the appropriate way of writing the Iron/thiocyanate equilibrium?
The second representation is more accurate because in water there can be no ``naked'' $\ce{Fe^3+}$ ion; as it goes into solution, iron combines with water to form an ion consisting of
a $\ce{Fe}$ atom surrounded by six molecules of water. It is the oxygens that form bonds with the iron atom, in a roughly octahedral arrangement: two oxygen
atoms in each coordinate direction, one on either side. The reaction
consists of the replacement of one of these coordinated water
molecules by the $\ce{SCN-}$ ion; now the iron has bonds to 5 oxygens
and 1 nitrogen atom.

The addition of $\ce{H2O}$ to the formula doesn't mean we are
diluting the solution with water.  It just means we're letting the
water that's  present anyway play a role in the equation, to reflect reality.

It's not clear to me what they are asking in (ii).  The colors change
dramatically when you mix $\ce{[Fe(H2O)6]^3+}$ (pale yellow) with $\ce{SCN-}$
(colorless); the result is deep red. Perhaps some context will help.  Read the discussion of the reaction in the text to see what they say about color intensity.

(iv) Again, what is the context?  If the text is about the spectrospic
measuring of the equilibrium constant, low
concentrations ensure that the reation obeys the ideal law.  In
concentrated solutions the law doesn't work quite right unless you
replace concentrations by activities, something that is only
indirectly measurable (it's whatever makes the formula work).

(vi) Again, we need to know the context.


No, not all equilibrium reactions need to be in a closed vessel. An
aqueous reaction that only takes a short time (so that for example
water won't evaporate appreciably, messing up the concentration) is
just fine in a test tube.