[RPG] Poison Spray + Saving Throws confusions

You extend your hand toward a creature you can see within range and project a puff of noxious gas from your palm. The creature must succeed on a Constitution saving throw or take 1d12 poison damage. (BD&D p98)

It's targeted at 1 creature. It affects 1 creature.

This is not an area attack, it's a directed attack. It's not a spell attack, so the additional creature doesn't grant cover, or even advantage.

Only the target is directed to make a saving throw or take damage so he is the only one affected.

It's worth noting that although this seems less directed than a ranged attack, the rule is similar. Basically everyone is moving all around in their space, you couldn't be shooting your poison at someone in an intervening space, but past them. The way I'd picture this is that those folks might get a whiff of the poison, but it's less of a puff and more of a directed jet at the target.

"Living Rules" is simply a way of talking about how they plan to make system updates and has no effect on a ruling in a case like this. A DM, I suppose, could use the freedom they are granted to make a ruling that additional creatures are affected, but it seems like the effects of the spell are intended for one creature only and other rulings may be quite unbalanced.

The Green Dragon's breath is described as:

Poison Breath (Recharge 5-6). The dragon exhales poisonous gas in a 90-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

It is "poisonous gas" the Dwarf gets advantage on the save.

Poison. Venomous stings and the toxic gas of a green dragon’s breath deal poison damage.

It is "poison damage" the Dwarf takes half damage (effectively 1/4 if saved 1/2 if not).

With respect to your comments on protection from poison, I think you are splitting hairs: it is poison so advantage is given on the save; the fact that the effect is to do poison damage rather than impose the poisoned condition is irrelevant to the save.

That said, the DM is the sole authority on rulings so, check with your DM.