Potash alum is potassium double sulphate of aluminium, i.e. a double salt as it contains more than one cation. Its formula is $KAlS{O}_4$. So, the cations present in potash alum are $K^{+}$ and $A{l}^{+3}$.

A salt formed from either weak acid or a weak base will get hydrolysed. Salts like $NaCl$, $KCl$ and $CaC{l}_2$ are formed from strong acid($HCl)$ & strong bases like $NaOH$ and $KOH$ respectively. Such salts formed from the reaction of strong bases and strong acids doesn't get hydrolysed.
${NH}_{4}Cl$ is formed from a weak base and a strong acid and therefore is an acidic salt.  
${NH}_{4}Cl$ is hydrolysed in water to give [$H^{+}$].
${NH}_{4}Cl$ + $H_{2}O$ ⇔${NH}_{4}OH$ + $HCl$
$HCl\to H^{+}+C{l}^{-}$
Here, $HCl$ is the strong acid formed and $N{H}_{4}OH$ is the weak base obtained.

Acids are defined by their ability to donate protons. Mineral acids like ${\text{H}}_2{\text{SO}}_4$ donate protons easily, so, they are strong acids. Organic acids like ${\text{CH}}_3\text{COOH}$ do not donate their protons so easily, hence they are weak acids. 

The given statement is false. Most of the metals react with acids. The metals which are more reactive than hydrogen can replace the hydrogen ion from an acid. For example, sodium, potassium can replace hydrogen from acids. The equation for the reaction of sodium and potassium with dilute hydrochloric acid is given below.

$2Na\left(s\right)+2HCl\left(aq\right)\to 2NaCl\left(aq\right)+H_2\left(g\right)$

$2K\left(s\right)+2HCl\left(aq\right)\to 2KCl\left(aq\right)+H_2\left(g\right)$

The metals which are less reactive than hydrogen cannot replace it from acids. For eg. Gold, Platinum etc. 

So the given statement is false.

Indigestion causes acidity in the stomach. We add anti-acids to neutralise it. The antacids are basic in nature and hence neutralise the acids. Magnesium hydroxide (milk of magnesia) is a commonly used antacid.

Salts of a strong acid and a strong base are neutral with pH value of 7. On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.
$\bullet$ $N{H}_{4}Cl$ is a salt of  weak base $\left(N{H}_{4}OH\right)$ and strong acid $\left(HCl\right)$. Hence, it is a acidic salt.
$\bullet$ $C{H}_{3}COONa$ is salt of weak acid $\left(C{H}_{3}COOH\right)$ and strong base $\left(NaOH\right)$. Hence, it is a basic salt.
 $\bullet$ $KCl$ is salt of strong acid $\left(HCl\right)$  and strong base $\left(KOH\right)$. Hence, it is a neutral salt.
 $\bullet$ $NaCl$ is salt of strong acid $\left(HCl\right)$  and strong base $\left(NaOH\right)$. Hence, it is a neutral salt.

When HCl gas is passed through a saturated NaCl solution, the number of ${\text{Cl}}^{-1}$ ions increases. The increased number of ${\text{Cl}}^{-1}$ interact with ${\text{Na}}^{+1}$ ions to form crystallised NaCl. Thus, the solubility of NaCl decreases and it is recovered in crystalline form. This phenomenon is called "common ion effect".