Is it ok to use aluminum for a busbar, or power distribution block?

Yes very correct but were also talking about 12 volts, not 15KV. You really think he will have problems with that piece of scrap? Yes there are better materials but I don't think he building a rocket here. I just think he is fine with he has made but everyone says to toss it and start over.
And as a side note if you have ever pulled 500 MCM then you would know that aluminum is much easier to deal with than copper, but not superior.