The Canadian mechanic, which may be of use here, is that the sides have a significant role to play, here. If it's close, we'd rather a few seconds of no-call/no whistle, than an inadvertant.
It depends greatly on the crew size, however. On seven-man, the R will line on the Q's throwing arm.
Otherwise, the R lines up on wide side (generally). On our four- and five-man crews, the wide-side official remains locked to the LOS until the ball crosses the LOS, and he/she is to rule onscreen attempts behind/over LOS, and suchlike. That individual may be able to assist the R on ruling a forward pass/loose ball.
This instance shows why it's important for the R to have a reasonable cushion. The further back the R is, the more the R sees.
I've been taught that in order to rule on those close-call passes in those situations, the R should move forward, so as to better get a sense of the line between origin of pass and termination point. To me, this does not necessarily prevent protecting the Q.