PanLin

You're so right 🙃 my b, I've updated my answer so it's actually about hemalurgy. Essentially, I think stubbornness (if we assume you mean it as a positive trait) is a more emotional than cognitive trait, so I'd go for zinc over copper.

Ah apologies, didn't mean to tell you stuff you already know 😅 but yes, this still applies to annihilation. From the wiki:
The energy comes from the conversion of Investiture and anti-Investiture. That wiki also states that the relationship is similar to matter and antimatter in real world physics. From the wiki page on Annihilation:
Whether the end result is matter, energy, new Investiture, or something entirely different, the rule of conservation still applies and nothing is truly destroyed.
Edit: ah sorry, I'm on the move and just realised I haven't answered your question. Please hold, I'll update in a sec
@Ati_Simp new answer for you:
What happens after annihilation
So, a couple of WoBs are useful here.
So, I'd say we can confidently say from this that matter, energy and Investiture being the same thing means they can be freely converted for however long without breaking conservation.
The bit I bolded seems to imply that this recycling would naturally happen even after the Investiture has been converted to energy, but it is a bit vague as to whether this is a natural process or he just means that it would naturally find its way back to the spiritual realm because of interactions with other agents and forces.
I also thought that purified Dor still returned to the Dor, which would mean that Investiture that's converted to energy still 'remembers' the Shard its keyed to and would end up back there, but the last WoB I linked makes it sound like Sanderson's shifting his perspective on that a bit.
Either way, it defo isn't permanently gone.
I really want to get myself to a convention to ask this question in person now 👀

So it's important to remember that Progression (and most forms of Investiture-based healing) doesn't technically 'heal' in the way you might assume. It changes your physical body to match your cognitive and spiritual perception of yourself.
The Lopen evidently still thought of himself as someone with two arms, so he was able to use Stormlight to manifest his missing arm and 'restore' his personal image of himself.
Kaladin's scar was sooooo integral to his perception of himself and where he was on his personal journey, so Progression couldn't heal it. Similarly, his own Stormlight rejected the tattoo because it was covering/modifying that scar, which went too strongly against his self-perception.
 
Some relevant WoBs (technically some minor spoilers for Stormlight and Elantris):
 

If you start adding copper to it, there would be a point where it stops acting like aluminum and becomes duralamin, but I'm not sure what exactly that point would be.
I view the alloys as tuning into a radio station—pure aluminum? Great. Perfect ratio of aluminum to copper? Duralamin, nice. Anything in between that is going to be the cosmological equivalent of static when you don't have the right radio frequency: either close enough to get the effect of one of those metals, or nothing at all (and give you a headache if you try to burn it). I think it's more of a scale rather than 'Suddenly Duralamin', if that makes sense?
That said, there are some non-allomancy alloys Scadrians have been experimenting with (eg: aluminum blended with things other than copper). There's a scene in Alloy of Law where they talk about an aluminum alloy that keeps some Investiture blocking while being stronger and less liable to corrode. From the wiki: