What about other non-Egyptians?

Just joking.
ما احنا اتفقنا إنك مصري يا جوش
The rule is really only about nationalities (and perhaps places of origin in general).
Yes, I think I prefer "place of origin".
Of course there may be some exceptions (such as شقة بحري/قبلي, but of course the normal agreement occurs as well), but maybe we should not group those in the same category as nationalities/places of origin, lest we get confused.
As we use both masculine and feminine with these 2 adjectives, I think the masculin one is more about the kind of aeration (?) the appartment get, rather than its location. In other words, we mostly say شقة
هواها بحري = well aerated, not very hot in summer, while a شقة قبلي/هواها قبلي is a hot one, even in winter.
My (perhaps wild) theory about this is that since we are talking about country/place of origin here, the nisba actually refers to the "origin," (الأصل) and not to the thing in question.
I don't think it is that wild. Actually it makes sense. Although we can't say for sure that it is
the correct explanation. Maybe we can never tell what the origin of this rule/practice. Just as we can't tell why do we use singular feminine adjectives with inanimate plurals in fuS7a.
When one wants to order, the object is not pluralized, as might be expected as per the grammatical rules (e.g. one would say "itneet 2ahwa min faDlak," and not say "2ahwateen min faDlak;" "khamsa kiilu mooz," and not "khamsa kiiluwaat mooz"). But this seeming incongruity can be cleared up if we think of the number as merely referring to the number of orders we want while the noun merely indicates what it is we want.
It's true we say number+kiloo, number+ahwa... but I can't think of an explanation either. Specially that with other orders we say: كوبّايتين شاي، طبقين رز/خضار/مكرونة ...
Abu Rashid, in my experience the ta marbuta is always pronounced clearly by Palestinians - at least clearly enough for me to hear it and notice it.
I agree, even if I don't hear much Levantine in general. But I know for sure that they make a clear difference between a feminine word, ending with taa2 marbuta, and a masculine word, without the taa2.
Colors, and most other descriptions, are not normally perceived as categories. In other words, when we think of categories of cars, we don't normally think of "red cards," "green cars," and "blue cars," but (at least in the Arab world) we do think of "German cars," "Japanese cars," "American cars," etc. The same goes for coffee.
I like the "label" explanation as well.
As for colors, we do use the masculine for some, and the feminine for others:
عربية سودة، بيضة، حمرة and بني (and not bonneyya) and فضي and not (faDDeyya=silver). It seems that some colors just sound better in masculine.
The basic categories are "Arab/Turkish coffee" and, well, "instant coffee" (which as I said we call "niskaffee" in Palestinian).
In Egypt too, we call it نيسكافيه .
Lastly, I have a question for speakers of Egyptian (and other dialects in which this occurs): What happens in the plural? What would you use to describe "3arabiyyat/sayyaraat"? "Almaan"? "Almaniyyaat"? "Almaani"?
As Josh said, we keep the masculine singular عربيات ألماني/ياباني/فرنساوي ....
No, we have our own gahwa 3arabiyya, which we usually just call ghawa (note the changed order of letters). It contains cardamom or cloves and has kind of a gold or light brown color, and it is never sweetened.
Do you really pronounce it as ghawa غوا ? or is it g-hawa?
This type of coffee, we call it ahwa m7awwega قهوة محوجة .