(a) When Rebbi Yehoshua says that on neither of the two cases is one warned against being Metamei it, only against eating it, he is referring to the wine that became Safek Tum'ah and on the oil that is going to waste.
(b) In defining the Isur of 'bal Tetamei', the Mishnah cites a case where a Nochri threatens a traveling Kohen that he will render Tamei all his Terumah loaves, unless he hands him one. According to Rebbi Eliezer - he is forbidden to give him one ...
(c) ... Rebbi Yehoshua - instructs him to place one on the rock for the Nochri to take.
(d) The Isur of 'bal Tetamei' that Rebbi Yehoshua is defining is - the prevention of all the loaves becoming Tamei, which the Kohen does by placing the one loaf on the rock (but no more).
(a) In similar vein, the Tana discusses a case where Nochrim demand that one hands them any one of a number of women (see Tiferes Yisrael), otherwise they will defile them all. Rebbi Eliezer and Rebbi Yehoshua agree - that they are forbidden to hand over one of the women (see Tiferes Yisrael).
(b) Rebbi Yehoshua concede to Rebbi Eliezer in this case - because of the principle 'Ein Dochin Nefesh Mipnei Nefesh (One may not push away one person in favor of another [and the same will therefore apply to where they demand any one person to kill, or they will kill the entire group])'.