Question:

If a player is not permitted to join a jam because she was not in position at its start, is she then allowed to join a Jammerless Jam that follows?

Answer:

No, such a player was not on the track in the previous jam, and so cannot be a part of the Jammerless Jam that follows.  The following rules logically extend to Blockers:

6.4.5.3 During a Jammerless jam, when one or both Jammers fail to be on the track when the jam starts, both teams will play a Jammer in the new jam.
6.4.5.3.1 No substitutions will take place.  The team(s) must skate short, however they are permitted to field a Jammer from one of the Blockers that played in the stopped jam.

Question:

If both Jammers are called off on penalties, but the jam ends before the second Jammer takes her seat in the penalty box, is there a Jammerless Jam?

Answer:

In the above situation, the Jammerless Jam rules apply, but new lineups are fielded because the previous jam has already ended. The original Jammers serve their time in the penalty box as Blockers, and the teams will begin the next jam short those Blocker positions.

Question:
When do Referees stop the jam-in-progress for a Jammerless Jam? When the second penalized Jammer is sent to the penalty box or when the second penalized Jammer reaches the penalty box?

Answer:
The Referee should stop the jam-in-progress and run the Jammerless Jam procedure when the second penalized Jammer sits in the penalty box, that is when her penalty time starts. One exception: when the first penalized Jammer has 10 seconds or less remaining on her penalty the Referees should not run the Jammerless Jam procedure.  A player in the penalty box should stand when she has 10 seconds remaining on her penalty.