1. There isn't a good alternative yet. The old legendaries aren't supposed to be easily replacable or else the grind for them would be useless. But the time skip will eventually add more equipment and give more options later. And no, you can't get back the blade Eagle from Aiyana.

2. You get Setsuna during Tsubaki's events, and you can improve it at the fire raid anvil in the Spire of Courage to Setsuna+. The legendary katanas Murasaki and Setsuni are unlocked in the time skip phase. More equipment will get added in later phases of the time skip but those are probably her best weapons for now.