Canonically? Yes. By the board? Not technically. But you have characters the resurrect despite how they died making it effectively impossible or unlikely, and more. I'm not innocent in that crowd. So while you can't parade around calling yourself Immortal, there is also no rule in Chaos forcing you to die or forcing you to have your death be permanent, or your resurrection make sense. So there's that. I'd encourage you though to just focus on the age-extending properties Alchemy offers. Which, like most of it's abilities, are varied and so vague as to give you a limitless tapestry to write within honestly.
As far as the species sub bit, Valiens there nailed it perfectly. Use your best judgment. Honestly, if you find yourself going 'should I sub this?' then the safest course of action is to just do it. Even if it gets denied for balance or what have you, that's invaluable feedback on how to adjust your story to better fit logic and our community.
Alchemy is an art that, unlike a lot of Force abilities, has a very wide open process on the 'how' really, and minimal hard and fast 'what' or concrete results. It's fun, so enjoy!
