Yes, this basically sums the issue up in a nutshell. No other grade has been expected to step in and cover for another role as part of their pay settlement, hence the difference in the response by the union representing conductors.
The demand that the RMT ballot it’s members should be seen for what it is, which is purely an attempt to avert the prospect of a strike during the summit. The action would have to be suspended in order to do so, however the prospect of strike action is the union’s biggest leverage here.
A lot of people attending the summit are staying in Edinburgh for the duration. The prospect of no train service between the cities during the summit is causing a lot of concern among both delegates and accommodation providers. There will be chaos if this isn’t resolved.