I have spent some time searching the rules forum and I'm unable to find a definitive answer to this question despite finding several threads that discuss it.
1) Can a tiny creature end its turn and occupy the same square as a medium creature? It must be able to do so or it would make it impossible for such a creature to ever engage in a melee attack since it would need to enter attack and leave each round.
2) If it can occupy the same square and end its turn there, does the medium creature flank the tiny creature? If not, what about if a medium creature has a tiny creature in its square and an ally stands adjacent to him. Does it flank it then?
Tiny critters generally have a reach of 0. You do not threaten with a reach of 0, and no threaten means no flank.
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Movement and size are pretty easy to resolve. Right in the combat chapter which is usually a better resource then debate threads.
Very Small Creature: A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.
Normally something needs to be three size categories larger/smaller but the smallest groups have a special exception I've italicized. So yes a tiny creature can enter and occupy the same space as a medium creature, so long as the medium creature isn't some kind of 5' cube creature taking up all the room in that square.
Flanking requires another enemy/opponent so No, you can't flank with yourself unless you're using some kind of feat/spell/special ability that allows it.
Flanking is normally considered to be center of square to center of square, if the line segment between those two points is through opposite borders of the opponent's square. Where the tiny creature is specifically is pushing things into undefined territory. To me it makes the most sense to center things so no, flanking wouldn't happen as the condition wouldn't be met. Your GM may decide differently.