Refereeing decisions

[JRugby2]

Isn't the ball dead between scrum resets? I would have thought it was a viable time for the clock to be stopped for substitutions just like before the first scrum is packed.

Yes but not in the same practical sense as pre scrum.

If we have a reset, the idea is to have that reset as quickly as possible. Once the failed scrum is stopped, it's generally a less than 10 second stop for the players to stand back up and the referee to deliver a message before the formation restarts. That's not enough time to facilitate a tactical sub.