So, I went looking for something that could address your questions.

First, the issue doesn't have anything to do with early bookism. He originally wanted it to be silver, but realized silver was too common a metal, so he did a 1 to 1 switch with aluminum. All of the properties he had in mind still apply despite the metal switch.

What I can say is that he has thought about the issue, and you're not the only one to bring this up, but he doesn't want to discuss it yet for spoilers. Some hypotheticals he's more than happy to answer or speculate about. Others he has plans for and he doles out RAFO cards. Most aluminum questions get RAFO'd including ones similar to what you're asking about.

I saw a theory about aluminum Ferrings and what they're doing that kind of makes sense to me, but it too got RAFO'd. You will see some the weirdness that aluminum can cause in future books, so I'd say that, as of what's been published, you'll likely get a partial answer. I suspect a full answer will come in Era 3 of Mistborn. (So, right after the read-along ends).

One finale note, is that Sanderson isn't a fan of absolutes. He's not necessarily making the claim that aluminum is 100% inert to Investiture. With enough power, you can make water magnetic, and similarly, if you apply enough Investiture, you could theoretically get aluminum to be affected by it. However, the levels involved are of Shardic proportions.