Recipes of Turkish drinks in Voltaire's Novel

First, let me clarify a linguistic point: Nowadays, both words "sherbet" and "sorbet" have entered the English language as loanwords. They both describe frozen desserts, and I have seen some people use them interchangeably and others making a distinction, e.g. that sorbet is dairy-free and sherbet has dairy. No matter how they are interpreted, sources about this meaning are not relevant to your question. It just so happens that the loanword's meaning has changed when entered English.

What you need is information about the word "sherbet" as understood in Turkey, not in English-speaking languages. And I am afraid that it is not a very informative word - it is a generic word for a sweet drink. I would actually go so far and translate it as "syrup" while noting that in modern Western countries, a syrup is seen as a concentrate to be diluted, while in Turkish cuisine, it is drunk straight (maybe at concentrations slightly lower than of the "simple syrup" found in classic western pastry books, but not too much lower).

Being that the word is so imprecise, there won't be a source referencing some kind of exact recipes for the drinks mentioned in that passage. Instead, you have to imagine people taking any combination of water, sugar (lots of sugar), and additional flavorful stuff, and drinking it. It is similar to a milkshake - what I throw in the blender is differnet from what my neighbour throws into the blender, and the "pistachio milkshake" I make today may have different ingredients and ratios from the one I made last week.

Dairy is not the first thing that comes to mind when thinking of sherbet, but there is also no sherbet police to prevent people from using it, so the kaymak (thick cream) was probably just added for more richness.

I wouldn't necessarily assume that the Mocha was drunk separately. Maybe it was, but maybe the specific example described was of a coffee based drink, the way people add espresso to milk to create a latte, or maybe they used a tiny bit of (brewed or ground) coffee as a flavoring. Just because it's unusual today, it doesn't mean that it has to be unusual across different cultures. Again, it wouldn't be a classic example for sherbet - the classic, purest form would be sugar and water, with maybe fruit juice as flavoring.

It seems not to get totally clear what exactly is meant here.

The term sherbet nowadays points more towards some sort of ice cream, than to drinks, as it usually describes a sorbet with very little milk fat added to give it a smoother texture. And while in Turkey Kaimak is a sort of (clotted) cream, in Greece Kaimaki is a mastic flavoured ice cream, that is similar to the Turkish speciality Dondurma, that traditionally also contains salep. Salep is gained from an orchid, that is nowadays threatened and should not be used anymore for this reason. The difference of Dondurma to ice-cream or gelato is that is very stretchy and chewy. The german translation of Candide is using the term Sorbet which definitley refers to ice-cream and not a drink. At the same time sherbet is derived from Persian sharbat, which in fact is a sort of drink, but from what I can find on this topic it seems to be unusual for it to go together with Kaimak and so I assume that it also might be possible that Voltaire added some fantasy and imagination to the description of the preparations as he did with so many other things in this great text.

I hope this helps to point you into the right direction(s) even if I can´t present you some historical sources. But you can find several recipes for traditional and contemporary Dondurma as well as sharbats easily on the net:

• https://www.dailysabah.com/food/2016/07/11/traditional-turkish-sherbets-to-cool-you-down-this-summer
• https://www.icecreamnation.org/2014/09/sultans-sorbet/
• https://turkishstylecooking.com/maras-dondurmasi-stretchy-ice-creamrecipe.html
• https://istanbulonfood.com/turkish-ice-cream-recipe/