If B5 is legal after one pitch, I'd love to see the rule/explanation that justifies that. None of the 3 of us thought that might be true.
The pitch to B5 made the previous batter legal. That was B3. Since B4 is on 3B and cannot be taken off the base, B5 is now the legal batter. One pitch to B5 and everything is water under the bridge.
Question - say he sends B5 to the plate for a pitch, and then, finally, defense appeals. B5's pitch has made B3 the legal previous batter. If defense appeals now, B4 is the proper batter - do we rule her out, remove her from 3rd, and put her at the plate?
No. A runner is never removed from a base to bat in the proper order. See crystal clear 7-2-C-4. With B5 in the box, everything is proceeding fine.
That doesn't seem to jibe with any other interps. Sidebar - can manager PR for B4 at 3rd and then bring B4 to the plate?
No. B4 is legally on 3B and cannot bat.
Another question might be this:
One pitch to B5 legalized B3's at bat. With B4 on 3B, B5 is the proper batter. But what if, with B5 up and having taken a pitch, B4 then gets called out on the look-back rule and is now not legally on base? (I'm sure that once B5's place in the box has been made legal [after a pitch], there's no going back.)