Can assumptions made during calculation of ka of buffer solutions be justified?
Answering your first question, $[\ce {H2O}]$ is constant because the square brackets indicate a concentration. Concentration is the number of moles of the solute on the volume of the solvent.

Except in this case, water is the solvent. So the concentration will always be constant, since the volume is proportional to the number of moles.

Besides, while dealing with the law of mass action, for liquids we don't take concentration, we take activity. Which is approximately one for water.

As for your second question, we basically have the following product:

$$\ce{([CH3COO- ]_{CH3COOH} + [CH3COO- ]_{CH3COONa})[H+ ]}$$

which can be rewritten as $(a + b)b$, where $b &lt;&lt; a$. Let's say $a$ was $1$ and $b$ was $0.0001$.

The actual value of the expression is $1.0001\times 10^{-4}$. Neglecting only the first $b$ will give $1\times 10^{-4}$, which is not too much different.

Neglecting only the second $b$ will give $1.0001$, and neglecting both will give $1$. Both of these answers are very wrong.

When a small number is added to a large one, the small number can be ignored. But not when it is multiplied.