I'm still worried about the rule clarfication concerning development/units when they are destroyed.

In Entropy42's rules summary the following Q&A is stated:

• Q: If I have a development that is also a unit (by some other card effect, such as Kairos Fateweaver), and that development-unit is destroyed, does it trigger "unit enters discard" effects such as Offering to Hekarti?
• A: The "unit enters discard" effect will only trigger if the development was a unit card itself. When cards hit the discard pile, they do so as whatever type they truly are, and not what type they may have been while in play. Cards which trigger off of a unit being destroyed, such as Garden of Morr or Doomsayer of Morr, will still trigger from the development-unit being destroyed, regardless of the development's true type.

Why is this situation different from the following?

• Q: Does a Dwarf Ranger trigger when a unit that is face down as a development leaves play?
• A: No. This is because it is a development leaving play not a unit. ( James' Ruling )

Does it has to do with "put into the discard pile" in case of Offering to Hekarti and "leaving play" in case of Dwarf Ranger? Does this difference justify the different handling?

In short, yes. Basicaly, when a development is destroyed/sacrificed, it leaves play as a development, but enters the discard pile as whatever it used to originaly be faceup. That does bring up the question what would a development under Kairos leave play as- would it trigger any 'unit leaves play' actions?

With Kairos in play, a development leaving play triggers both "development leaving play" and "unit leaving play" triggers, as it is both a unit and a development. When it hits the discard pile, it then triggers any "enters discard pile" effects for its true card type.