So what do you plan to do when the third batter is due up in that inning and she's still standing on 2B?

Who is the first batter due up in the TB inning? I assume it would be B4. As Altor asked, unless B6 has scored already (or been put out), she will still be on base after B5 bats. What then?

I could be semanticly particular and say that "the last batter in the lineup for the inning" by referencing the inning rather than the whole game would mean whoever is left existing in the batting order, but I won't as you are correct.

The last completed at bat was still by B3. It just happened to be filled by an improper person in that spot.
B3 goes to second.

If you take the "last person with a completed at bat" as you do above, you would have a problem in the following situation:

B1 out
B2 out
B3 out
Sub player X for B3 (for the defensive half inning)
By your logic, you are saying that B3 is up. Actually its X for B3. Ecssentially meaning the person occupying the spot in lineup where last completed at bat occurred.

No, B3 is out because she did NOT complete her at bat.

No... I don't. Alex does - my example was an attempt to show why that could be incorrect...

OOOOOOOOOooooooooooooooops....
I was reading after B2 out: B3, B4 and B5 on the bases when B6 hit that homer.
However, you're talking BOO here. Then we have:

Sec. 2. BATTING ORDER.

d. The first batter in each inning shall be the batter whose name follows that of the last player who completed a turn
at bat in the preceding inning.
EFFECT - Sec. 2c-d:
Batting out-of-order is an appeal play that may be made by the manager, coach, or player of the defensive team only.
The defensive team forfeits it's right to appeal for batting out-of-order when all fielders have clearly vacated their normal fielding positions and have left air territory on their way to the bench or dugout.
1. If the error is discovered while the incorrect batter is at bat
(a) The correct batter may legally take his place, and assume the ball and strike count of the incorrect batter.
(b) Any runs scored or bases run while the incorrect batter is at bat shall be legal.
2. If the error is discovered after the incorrect batter has completed his turn at bat and before a legal or illegal pitch has been made to another batter
(a) The player who should have batted is out.
(b) Any advance or score made as a result of the improper batter becoming a batter-runner shall be nullified.
Any out that is made prior to discovering this infraction, remains out.
(c) The next batter is the player whose name follows that of the player called out for failing to bat. If the next player was the incorrect batter who was called out, go to the next person in the line-up.

Then yes, you are correct B4 is at bat. The homer by B6 is nullified and has not a complete turn at bat. At the score-sheet will show up that B3 has however a complete turn at bat. Even while she didn't show up at bat...

I must read much s l o w e r, when it's in the Queens English. Sorry for the miscommunication and for making myself look like an idiot.

Mike, there is a slight difference in "not completing your at bat" and having a complete turn at bat: B3 was out for NOT BATTING. That gave her a complete turn at bat...

Think about the logic behind the rule.
What we are trying to avoid is to have a runner on base when it is their turn to bat.
By using the batter preceding the one who is due up, the odds are reduced to the minimum.

I agree and used to define just as that. Unfortunately, there would still be questions as everyone thinks they have a TWP that would circumvent it.

There are multiple ways to define it, unfortunately it seems that no matter how simple it is offered, there is always someone who doesn't get it or doesn't want to get it.

In Oregon it depends on what the individual leagues decide. Most start it in the 10th. One or two do it in the 8th. All the leagues use the 10 after 5 run rule.

I had one 10-inning game last year and one this year. The tiebreaker did its job since both times the game ended that inning.

EDIT: If two teams from different leagues are playing each other we use the home team's league rules. In state tournament games the tiebreaker starts in the 10th and the run rule is 10 after 5.

Congratulations on the game assignment. Just wondering, how does a "newish umpire" who is not sure of the rules obtain such a choice gig?

At the risk of speaking on behalf of the OP, I would bet that this is probably a smallish weekend tournament sponsored by a school, not a playoff assignment.

NFHS SP uses the tie-breaker rule? Do any states actually play slow pitch as a high school sport?

NFHS SP requires state adoption of game ending rules, just like FP.

Last I heard, Mississippi plays NFHS slow pitch in the fall (FP in the spring).

That very well could have been the case.