You should say the blessing on the date. Since the verse in Deut. 8:8 praises the Land of Israel with 7 species (2 grains and 5 fruits), the Sages said that when you make a blessing "Ha’aitz" on different fruits -- and you plan on eating one of the fruits of the 7 species as well -- the blessing should be recited on fruits of the 7 species.
Even within the 7 species, the order is determined by how closely that specie appears in the verse to the word "land." The verse reads: "A land of wheat and barley, and vines and fig trees and pomegranates; a land of olive trees and [date] honey." The order, therefore, is: wheat, barley, olives, dates, grapes, figs and pomegranates.
Furthermore, the blessing is said over fruits of the 7 species even when another fruit requires the blessing "Sheh-hechiyanu."
Source: Shulchan Aruch O.C. 211:1, 4.
No. The Sages only said that the original blessing should be recited over fruits of the 7 species, thereby making the blessing more "special." Beyond that, however, there is no requirement to eat them in any special order.
Source: Talmud - Brachot 40b
The 5 fruits require still their own bracha of HaEitz! This is because since they are more prestigious and you didn't have them
specifically in mind at the time of the bracha, they were not
covered by the original bracha.
Source: Shulchan Aruch O.C. 211:5; MB 33.
No. Even though the halacha requires you to say the blessing over the fruits of the 7 species, nevertheless if you had them specifically in mind when making the blessing on another fruit, that blessing is still effective for fruits of the 7 species.
Similarly, if you made the blessing "Shi-hakol" (by mistake or intentionally) on food requiring a different blessing, the blessing is still effective.
Source: Shulchan Aruch O.C. 211:5; MB 33.
No, because since they are in front of you at the time of the bracha, it is as if you had them in mind at the time of the bracha and they are covered.
Source: Shulchan Aruch O.C. 211:5; MB 32.
No. As long as there remains some fruit in front of you for which the blessing is still valid, the original blessing continues on any other food (of the same blessing) which is brought afterwards.
If, however, all the fruits were finished (or you decided to finish eating), and only afterwards the apples were brought in, since at the time of the blessing you didn't specifically have in mind to eat apples as well, they will need a new blessing. It’s best to always have in mind that the blessing be effective for whatever similar-type food you might wish to eat later.
Source: Shulchan Aruch O.C. 211:5; MB 32.
No. Even though we say that the blessing continues as long there remains some food for which the blessing is valid, this only works for food that is similar to the original food. Therefore, it does not work from solids to liquids.
Source: Shulchan Aruch O.C. 211:5; MB 21.
Yes. Since you left the building, even with full intention to come back and eat more, this constitutes a Shinui Makom -- a change of location which terminates the original blessing.
Source: Shulchan Aruch O.C. 178:5-6; MB 45.
9a. Yes. Because in the case of the fruits of the 7 species where you ate a kezayit, it is a halachic doubt whether Shinui Makom terminates the original blessing. Therefore you are in a situation of safek bracha. You should preferably remove the doubt by either first reciting an after-blessing, or by hearing the before-blessing from someone else.
Source: Shulchan Aruch O.C. 178:5-6; MB 28.
9b. Yes. Because in such a case, Shinui Makom does not affect the before-blessing, and you may therefore continue eating without a new blessing -- even if upon returning you found that the other people had already left!
Source: Shulchan Aruch O.C. 178:2; MB 15
10. Yes, when dealing with a "snack" (as opposed to a bread meal), a firm decision to stop eating terminates the before-blessing and a new blessing is required.
Source: Shulchan Aruch O.C. 179; "Biur Halacha" s.v. "Ain."