Comparison of acids to dissolve phosphates
N, or Normality is a way of standardizing the # of acid equivalencies a particular acid has across multiple acids.  Acids that have only 1 acidic hydrogen for example $\text{HCl}$, the Normality will equal Molarity.

Normality is defined by:

$$
Normality = \frac{c_i}{f_{eq}}
$$

Where $c_i$ is the molar concentration of the acid, and $f_{eq}$ is the equivalency factor.  In the cases of something like $\text{H}_2\text{SO}_4$, the equivalency factor is 0.5 because it is a diprotic (two proton) acid.

This test also is showing the effect of weak vs strong acids.  $\text{HCl}$ is a strong acid, so it dissociates essentially 100%, while citric is a weak acid.  A weak acid exists in equilibrium between the dissociated acid species, and the neutral species.  This equilibrium is defined by the acid-dissociation constant $K_a$, or it's logarithmic version $pK_a$.  

For citric acid though, it is triprotic, meaning there are 3 acidic hydrogens, so it's $f_{eq}$ value is $1/3=0.3\overline{3}$

Your numbered questions:

1)  The speed is being measured to a certain point.  However, the chance is that all of these reactions are complete at this point.  The hour timepoint is to normalize everything to the same time frame.  But as indicated, the reactions are likely complete at this point.  What is really being measured is your second point.

2)  Absolutely, the quantity of acid needed to dissolve the material is being tested.  As you use more and more volume of acid, you are providing larger amounts of acid to the reaction.  This helps it dissolve more of the powder (as there is more acid now present), and also increase the rate of the reaction.

They standardize the reaction to normality because if they used molarity, 1 mole of citric acid has 3 acid equivalents, while $\text{HCl}$ has only 1 acid equivalent.