How does this wording differ from "Whenever [something happens] the first time each turn," like we've already seen for years on cards like Jetting Glasskite and Vengeful Warchief? Does it differ, even in corner cases, or is it just a templating update?
I love this card. I've always loved this card. Balduvian Horde and Helm of Obedience and Force of Will all seemed overhyped; this was the face card of Alliances to me. Of course, I feel silly now for being so quick to trade away all my extra FoWs, but that's how it goes.
It is clear whether that last clause means "count legendary creatures and all planeswalkers" or "count legendary creatures and legendary planeswalkers"?
No, only the original Jace counts. From his rulings:
The tokens created by Jace’s last ability don’t have the legendary supertype. If another object becomes a copy of the token, that copy also won’t be legendary.
This misses the point of thatmarkguy's question entirely. He knows that the copies of Jace are non-legendary planeswalkers; there's no other reason to even bring up the card. He's asking about the genuine ambiguity on this card, as well as on Mox Amber and on the reminder text printed on legendary sorceries like Urza's Ruinous Blast: does "legendary creature and planeswalker" mean "planeswalker and legendary creature" or "legendary creature and legendary planeswalker"?
The correct interpretation is almost certainly "legendary creature and legendary planeswalker", per the first ruling on Urza's Ruinous Blast and the comprehensive rule for legendary sorceries ("205.4e Any instant or sorcery spell with the supertype "legendary" is subject to a casting restriction. A player can't cast a legendary instant or sorcery spell unless that player controls a legendary creature or a legendary planeswalker."). On the other hand, the rulings for Mox Amber are just as ambiguous as its rules text, there's no comprehensive rule covering this wording that I can find, and the reminder text printed on legendary sorceries is just that - reminder text.
Still a bit redundant, "it fights up to one creature" or "you may have it fight a creature" would have it covered.
No, it targets the creature. So if it were "you may have it fight target creature an opponent controls", and there was no legal target at announcement, the trigger would be removed from the stack and you wouldn't gain 3 life.
Similarly, if you choose a target and it's illegal at resolution, the entire trigger is countered and, again, you don't gain life.
The correct interpretation is almost certainly "legendary creature and legendary planeswalker", per the first ruling on Urza's Ruinous Blast and the comprehensive rule for legendary sorceries ("205.4e Any instant or sorcery spell with the supertype "legendary" is subject to a casting restriction. A player can't cast a legendary instant or sorcery spell unless that player controls a legendary creature or a legendary planeswalker."). On the other hand, the rulings for Mox Amber are just as ambiguous as its rules text, there's no comprehensive rule covering this wording that I can find, and the reminder text printed on legendary sorceries is just that - reminder text.
Similarly, if you choose a target and it's illegal at resolution, the entire trigger is countered and, again, you don't gain life.