You need to take into account dosage.

They usually have to go through it again, FDA clinical trials (phase i, ii, iii) with different dosage to see what's effective. Increase in dosage can increase chances of side effect or create new ones.

There may be laws to get around or fast track that but pharma company have a dedicated department to figure that out. Just like that one company that causes opioid epidemic.

If it's a drug that blocks entry of the virus into human cells, wouldn't it be ineffective on already infected human beings?

If it works it will block infection of new cells. Already infected cells will die and be shed.

Biologists can correct me if I'm wrong, but it seems like fighting infection is a war of attrition. So anything you can do that shifts the balance helps.

Some infections might be a war of attrition, but it's not something you can generalize. Fundamentally, stopping entry would just prevent the virus from infecting more cella in the body (which could worsen symptoms) - and coronavirus is more of your standard fast invasion - burn bright and strong before being eliminated style infection than a chronic slow burn, so stopping that fast invasion could help.