As Taff, Pegleg and crossref have all indicated, I think the wording of this law leaves a lot to be desired. This is World Rugby's second attempt at it too, though I am not sure that it is really any clearer than the first attempt was.
Certainly there does seem to be an interpretation by many coaches that the scenario outlined above would automatically trigger a move to 7-man scrums for each side, but I have never been convinced of this and raised the matter with the society in order to get a view. The upshot was as follows:
The key point is the reference to a team being 'incomplete and it cannot field eight suitably trained players in its scrum, the formation must be as follows:
Red are indeed "incomplete" as a result of a forward receiving a yellow card. However, Red are also in a position (by utilising interchanges) to field eight suitably trained players in the scrum. We know from the information given in this scenario that there are two "suitably trained" players on the bench. (Though they could equally well have been on the field and playing in the back line. The key point is that the coach has identified prior to the match that he has "suitably trained" replacements available.) One of these replacements should come on (or into the pack, if they are already on the field and playing in the backs) at the next scrum and at this point another player be nominated to leave the field. (In much the same way as we would handle a scrum at senior level if a front row player was in the bin at the time). The regulations at U19 relate to the front five, so you could argue that any of the backs could come into the back row of the scrum. However, I think it would be unwise to insist on this for the reason that both Taff & Pegleg allude to i.e. that the replacement player should be experienced as a forward in some capacity at least. So, if it is accepted that they have a suitable forward replacement on the bench or elsewhere on the field, then the scrums should remain as 8-a-side and the offending team plays one short in the backs.
Obviously, if the situation were one where there were no "suitably trained" forwards available (because the team started the match short, or had already lost their suitable replacements to injury or dismissal) then scrums should be balanced accordingly with reduced numbers.
This interpretation of the law variation makes perfectly good sense to me, as it rightly addresses the safety aspect, but without allowing the offending team to mitigate the impact of its YC offence by keeping a full back line in situ and, therefore, not granting the non-offending team the opportunity to exploit the additional space.
