(a) 'Zeisei Meisik' refers to olives picked by a Yisrael, 'Zeisei Nokef' - to olives picked by a Nochri ...
(b) ... which are Patur from Ma'asros.
(c) Similarly - 'Invei Batzir' (with reference to grapes from regular bunches) are subject to Ma'asros, whereas 'Invei Olelos' (grapes from incomplete clusters, which already belong to the Aniyim) are not.
(d) If the owner finds himself with a mixture of Zeisei Meisik and Zeisei Nokef (see Tiferes Yisrael [or Invei Batzir and Invei Olelos]), assuming that ...
1. ... he has other Tevel olives (or grapes) - he should separate all the Ma'asros from the latter to cover the 'Zeisei Meisik' (or the 'Invei Batzir') as well.
2. ... he has none left - he should separate from the entire mixture Terumah and Terumas Ma'aser (to give to the Kohen [see Tos. Yom-Tov] after separating Ma'aser Rishon), and from the remainder one tenth is Ma'aser Rishon and a second tenth Ma'aser Sheni (which he redeems [both in accordance with the amount of Zeisei Meisik (or Invei Batzir), or in the appropriate years, Ma'aser Ani.
(a) The basis of the Tana's distinction between Terumah and Terumas Ma'aser and the other Matanos - is the fact that eating Terumah and Terumas Ma'aser is an Isur (for which one is even Chayav Misah), whereas the other Ma'asros (including Ma'aser Sheni once it has been redeemed) are purely monetary issues (to which we apply the principle 'ha'Motzi me'Chaveiro, Alav ha'Re'ayah').
(b) Ma'aser Rishon is mentioned twice - because the first time he is obligated to separate it in order to separate Terumas Ma'aser from it, whereas the second time he is only obligated to give the Levi according to the amount that is actually Chayav.
(a) If the owner takes yeast from a dough made of wheat-flour and kneads it with a dough made of rice - he will be Chayav Chalah if the dough tastes of wheat (like we learned earlier in the Perek). Otherwise, it is Patur (see Tos. R. Akiva Eiger).
(b) The Mishnah reconciles this with the principle (that governed the previous Mishnahs) 'ha'Tevel Oser Kol Shehu' - by confining the latter to 'Miyn be'Miyno' (when it is mixed with the same species), but 'Miyn be'she'Eino Miyno' becomes Batel only when it leaves no taste (which is usually one in sixty).
(c) The reason for this distinction - is based on the source of 'Tevel Oser Kol Shehu', which is the principle 'ke'Hetero Kach Isuro', meaning that the Shi'ur Isur of Tevel is based on its Heter (and even one grain [min ha'Torah] exempts the entire pile of produce), a principle that is confined to grain belonging to the same species, but not to Miyn be'she'Eino Miyno (see Tos. Yom-Tov DH 'ha'Tevel Oser' and DH 've'she'Lo be'Miyno' and Tiferes Yisrael).