as Far as Flanking/adjacent, the question is improperly worded. While you do not need to be adjacent, you DO need to THREATEN. This is kind of important, as you're other questions mention a PC with reach.
Example: 2 PCs are in a 5 foot wide corridor, a Rogue in front, and his Bard friend right behind him wielding a longspear. Suddenly, An Orc charges the rogue in the surprise round! Everyone rolls initiative; the Bard goes first, then the Rogue, and last the Orc. The Bard readies an action to trip the orc when the Rogue gets into flanking position, and ends his turn. The Rogue successfully uses acrobatics to tumble through the Orc's square at CMD + 5. The rogue and Bard are now flanking the Orc, even though the Bard is not adjacent. The Bard will now attempt to trip the Orc with his readied action at a +2 for flanking, rather than a -4 for soft cover from an ally. Once the Bard uses his action, the Rogue can now sneak attack the (possibly prone) Orc.