It’s a model with a screwed aluminum case, meaning I cannot unplug the battery when I don’t need it. How long does it last?
Considering that it’s a 10-year old battery, it would’ve already degraded significantly, even if it wasn’t being actively used. If battery life is important to you, you should first consider getting a replacement battery.
As to how long it’ll last, that can only be answered by you as it depends on which distro you’re going to use, what DE/apps you’ll run, and your actual usage patterns/workload.
Is there anything apple does better than linux that deserves I don’t nuke it?
Adobe CC, or multimedia stuff in general. And the fact that you can run MS Office natively. But if you don’t care about that, then just nuke it.