Named Mob Disappeared

I name tagged a zombi as "zombi". then it was there until today, for 2 days. then, i broke a couple of nether portals. then, i went to the zombi's place (no sun light) and it was gone.  I have several issues actually; 1-) Too much slowness. Game is so slow (Lenovo Y50-70)... 2-) I can pass through unpassable blocks (dirt, glass, chest, etc.) time to time for only horizontal direction. 3-) villagers can breed for several times without feeding them after once fed. (i am not so sure about the problem since i give 2 stacks of meals and they take all) 4-) the most boring one, nether portals does not link properly. That was why i broke them at the first place.   5-) the orginal one, dissappearing mob.