Activating a tile.. moves tile back
When a tile unit is completely removed from the board, you flip the tile bloody side and then.. moves tile back?
Correct. Per the OL book, page 4, the tile is flipped facedown (bloody side up) and moved to the end of the River.
When u want to Respawn figures that where completely killed.. u flip the tile back from the bloody side.. place the units paying the cost and they get activated paying the activation cost.. move tile back?
Almost. When you reinforce "dead" units, you flip the bloody-side-up card to bloody-side-down (in other words, bloody tiles only remain bloody so long as all of their minis are off the board). Bringing in reinforcements does NOT activate those units, so the newly-flipped tile stays where it is. Thus if you want to immediately activate those units after respawning them, you'd need to use your second activation to do so.