There is no chemical difference between adding salt early or late in the cooking process. However, if you salt just before eating, you can take advantage of textural differences between different types of salt. (Kosher salt is flakier and so gives a burst of saltiness that is quite pleasant.) (Some people will say that you should salt earlier to bring out flavors more, but once the salt is dissolved in solution, the chemical effect is the same.)
In general, fresh herbs tend to be added later in the preparation -- with the exception of parsley or dill added to soup early on to flavor the broth. However, if the herbs are added early, they must be removed, since they will lose all their flavor. (The broth will gain flavor, but the herbs will be exhausted.) You'll maintain the bright flavor if you add them late.
Dried herbs tend to react better to longer cooking, with bay leaves in particular requiring lots of time to render out their flavor.
As far as pepper goes, there's a fantastic answer to this question that goes into the chemistry of the volatile compounds in the spice. In general, I find that I prefer adding pepper later in the cooking process.
Garlic, ginger, onions, and other aromatics can be cooked a long time, although the flavor of alliums will change enormously as you cook them. (Garlic, in particular, will change enormously from a sharp flavor when raw or quickly cooked to a rich sweetness when cooked a long time.)
There is no standard spice mix for Doner Kebabs. This generally applies to any food in any part of the world. There can be a common mix, but as you have experienced, they can be quite specific to certain areas of the world
A major factor for noting a common spice mix is the global food supply industry. What happens in general is that food retailers buy bulk spices from their local food wholesaler. In many regions there is little competition for food wholesaling. Food wholesalers pre-mix ingredients for popular food items in suitable sizes. So in one location, other than a few "independent" or "food artisan" places, everyone seem to taste the same
I have seen this with some hilarity in my own country. We have one main food wholesaler for each of the two main geographic areas our country is divided into. For Easter, hot cross buns have a strong spice flavour, but most bakeries seem to just use the local food wholesalers pre-mixed spice. The two main food wholesalers spice mixes have entirely different flavour profiles. So people travelling into the other region think that the hot cross buns are a bit bland. Both are fine, but just quite different
For Doner Kebabs, this could be just a single source of dry spice mix, or maybe they all get pre-assembled gyros from a specialist meat supplier whom uses their own spice mix? As the popularity of Doner Kebabs rises, other meat suppliers getting into making gyros may just copy this spice mix for lack of knowledge or originality. The latter is common in many western countries
So a polite ask at your local Doner Kebab vendor to whether they make their own gyro, or buy a spice mix from a wholesaler may solve the problem?
Best Answer
Without knowing the exact spice / herb / compound in question, I can't give you a definitive answer, but your hypothesis is right on the money.
I'll try not to go completely "chemistry teacher" on you, but water is often referred to as "the universal solvent" for good reason. Given enough water, it can dissolve just about anything. (There's gold dissolved in seawater. Not enough to be profitable to harvest, but enough to be measured.) Just as an example: rosmarinic acid (one of the compounds - perhaps not surprisingly - found in rosemary) is classified as "slightly soluble" in water.
If you look at the diagram of the chemical structure (that I've brought in from wiki), all those lines are shorthand for carbon-carbon bonds - which typically mean a chemical dissolves easily in oils.
The OH groups are alcohols (but not the getting drunk kinds), which typically make it easier for a chemical to dissolve in water. The alcohol group, the carbon between, and the double bonded oxygen combined make something called a "carboxylic acid" (hence the acid in the name).* Carboxylic acids are also water soluble. So we have a chemical that can dissolve in both oils and water.
When we talk about infusing oils, the amount of the essential oils, etc that go in is usually pretty small, compared to the overall volume of the oil. So, if you add just a little water, a chemical that is even "slightly soluble" can make the jump into the water and leave your oil sadly lacking.
* Vinegar is acetic acid dissolved into water; acetic acid is another carboxylic acid.