Smaller molecules will absorb faster
Isn't alcohol a larger molecule than caffeine?
Your body knows very well what sodium chloride is
It has no idea and does not care whether it comes from the ocean or the ground.
Today, salt mined from the ground needs less purification than salt from the ocean. That's how wrecked with pollution the oceans are
Sooo, table salt from the baking aisle of the grocery store will have exactly the same effect as sea salt.
As far as the coffee itself there are small differences among the brands. I don't think those differences are enough to have any effect in an enema
Actually, no!
Smaller molecules will absorb faster
If the characteristics of absorption were dependent only on the principles of a molecular sieve where molecular size was the only determinant, the answer might be "maybe." But that ignores so many other drivers. For example, concentration gradients, the polar/non-polar characteristics of a particular molecule, and whether a particular compound is "keyed" to certain cellular functions are all factors at the micro level.
Isn't alcohol a larger molecule than caffeine?
Not only NO! But HELL NO!
Ethanol (the alcohol I assume you are talking about) has a molecular formula of C2H5OH.
Caffeine is C8H10N4O2. It is massive compared ethanol. And one of the interesting things about a number of nitrogen containing organics is how easily they are absorbed through the skin. Has nothing to do with the "size" of the molecule. Has everything to do with the highly polar nature of the compound due to the nitrogen in the molecular structure.
Your body knows very well what sodium chloride is
Actually, it doesn't. There are biochemical balances of certain electrolytes (that would be sodium to name one of several critical electrolytes that include others like potassium and calcium) that the biochemistry automatically handles to balance within a range that is controlled by the necessary biochemistry of the cells. That your body might not react differently to common table salt compared to sea salt is a reasonable statement from the standpoint of the basic sodium chloride. However, there are two different molecular weights of sodium chloride based upon whether the chlorine is the stable isotope of atomic mass of 35 or 37 (there is but one stable isotope of sodium). And it is possible that there are slight differences between "sea salt" and salt that is mined in terms of ratios between the two stable salt isotopes.
One other note is that "table salt" adds a small amount of dextrose to the salt to make the salt "taste" more like salt. And many table salts are iodized (for a completely different reason associated with your thyroid function). One other thing; sodium "anything" has a high degree of solubility in water.
As for caffeine in coffee; light roasted coffees tend to have more available caffeine per gram compared to medium and dark roasted coffees. This level of roasting affects both the flavor and the amount of caffeine per gram of coffee brewed that might be contained on a given brewing cycle (mg/ml).
I don't know where you get your ideas from, but this is pretty basic chemistry for most college level graduates with a science, math, or engineering background.