The info for Tam Tara hard means the minimum ilevel required to enter is i70, and if you are above i110 your ilevel will be synced to i110.

For the rest, as I understand it, if you go into lower level dungeons your character level will be synced and all stats appropriately lowered, and some abilities locked out. Your gear's stats will also be synced to a level appropriate ilevel.

As for avoiding level syncing by not going through DF, I'm not quite sure what that means. If you go to the actual entrance of the dungeon it opens the DF window anyway. With the new expansion, though, we now have the ability to turn off level syncing if we go in with a preformed party. If you queue to get matched with randoms you'll always have to be level synced (if the dungeon requires it).