The binary operation is defined as p q = (pq)/(7) + p + q.

The identity element $e$ satisfies $p \oplus e = p$ for any real number $p$.

$$p \oplus e = \frac{pe}{7} + p + e$$

Setting this equal to $p$:

$$\frac{pe}{7} + p + e = p$$

Subtract $p$ from both sides:

$$\frac{pe}{7} + e = 0$$

Factor out $e$:

$$e \left( \frac{p}{7} + 1 \right) = 0$$ $$e \left( \frac{p+7}{7} \right) = 0$$

For this equation to hold for all $p$ (except $p=-7$), the identity element must be $e=0$.

Let the inverse of -2 be $x$. By definition, $(-2) \oplus x = e$. Since $e=0$:

$$(-2) \oplus x = 0$$

Using the definition of the operation:

$$\frac{(-2)x}{7} + (-2) + x = 0$$ $$-\frac{2x}{7} - 2 + x = 0$$

Multiply the entire equation by 7 to eliminate the fraction:

$$-2x - 14 + 7x = 0$$

Combine like terms:

$$5x - 14 = 0$$

Add 14 to both sides:

$$5x = 14$$

Divide by 5:

$$x = \frac{14}{5}$$

The inverse of -2 is $\boxed{\frac{14}{5}}$.

Using the definition $p \oplus q = \frac{pq}{7} + p + q$, with $p=m$ and $q=5$:

$$m \oplus 5 = \frac{m \cdot 5}{7} + m + 5 = \frac{5m}{7} + m + 5$$

Using the definition $p \oplus q = \frac{pq}{7} + p + q$, with $p=m$ and $q=7$:

$$m \oplus 7 = \frac{m \cdot 7}{7} + m + 7 = \frac{7m}{7} + m + 7 = m + m + 7 = 2m + 7$$

$$\frac{5m}{7} + m + 5 = 2m + 7$$

Combine the $m$ terms on the left side:

$$\frac{5m}{7} + \frac{7m}{7} + 5 = 2m + 7$$ $$\frac{12m}{7} + 5 = 2m + 7$$

Multiply the entire equation by 7 to eliminate the fraction:

$$12m + 35 = 14m + 49$$

Subtract $12m$ from both sides:

$$35 = 2m + 49$$

Subtract 49 from both sides:

$$35 - 49 = 2m$$ $$-14 = 2m$$

Divide by 2:

$$m = \frac{-14}{2}$$ $$m = -7$$

The truth set is $\boxed{\{-7\}}$.