Could Alchemist use a couple of those Youth Potions to reverse engineer the formula?
I can actually answer this!
No.
As much as he claims to be a wizard, he's not actually an apprenticed or trained wizard/alchemist. He's missing a lot of the basics and fundamentals that would be necessary to do that.
Arcane Sight is actually helping him to learn more about spell forms and structures than the various spells or magical skills he's picked up. Given some time to watch actual magicians (Kaldur, Giovanni, and Aquaman come to mind) manipulate and tweak their magic and he could start to do more than just fill magic into a spell structure and let it go.
Hell, at the moment the only spell he can actually modulate is Create/Destroy Water because it's got power controls built into it, and the skills Transmutation and Convert.
As to the Elixir itself... No recipe has actually been provided in canon, but if I was going to explain its ingredients and properties;
The ginseng juice would offer longevity, the small heart likely came from some small animal, perhaps a salamander for the property of regeneration and have that powered by Greater Restoration. The various venomous animal components trap and then invert some kind of aging curse, perhaps an imitation of a Ghost or Banshees Horrifying Visage.
I really don't think Alchemist needs more ways to power. He already has the largest toolbox of powers in DC.
You're pretty much right. His goal isn't power at this point so much as defense. He's got enough gun, and he's mobile as all hell but if he got pinned down somehow (Dimension anchor would no-sell teleportation) then he goes down pretty easy. Duffy wasn't able to hurt him much by way of her weapon of choice but someone with a more powerful weapon landing a sneak attack would wreck him because he's not built to tank it.
You're fine. I love good questions and suggestions.
I've got an idea for Nabu, no small part of which comes down to the fact that his depiction in Young Justice
doesn't make sense!
You'll see more when I actually get to it, but to offer a teaser to the idea:
The scales are imbalanced for far too long a time if we go by the YJ timeline. Something else must be at play, but the question is what?