Antacid aluminium hydroxide

1.
By some Google-ing, this site suggests that aluminium can cause relaxation of the smooth muscle of the small intestine. This slows down the peristalsis of the intestine, leading to slower transport of the digested food.
The longer retention time of the digested food also leads to more water being absorbed. The digested food becomes drier.
These 2 factors cause the constipation.

Magnesium hydroxide has the opposite effect. Mg is poorly absorbed in the small intestine. The unabsorbed Mg remained in the digested food in intestine leads to build up of osmotic (water) potential. Water is retained, which may cause diarrhea.
Mg can counteract the constipation resulted by Al.


2.
The water is called "water of crystallization". Water molecules stays within the lattice of the ions.
Number of water of crystallization depends on the nature of individual compound.

The number of water of crystallization can be determined experimentally. A dry hydrated salt is weighed, then dehydrated in an oven at suitable temperature. The dehydrated salt is weighed again.
With molar mass of the anhydrous salt and the loss in mass (of water), the number of crystallization can be calculated.

Let Mr be molar mass of the anhydrous salt, n be number of crystallization:
mass of hydrated salt / (Mr + 18n) = mass of dehydrated salt / (Mr)