When did we see a Romulan who wanted Earth attacked, much less destroyed? All of Shinzon's allies appeared to be solidly against the idea.
What they wanted Shinzon to do was to proceed with "an attack on the Federation", with "defeating the Federation", apparently by rallying a joint army such as envisioned by the Romulan conspirators speaking to the Senate in the teaser. Shinzon's enraged sally from Romulan orbit towards Earth does not appear to be part of any plan agreed on with the conspirators.
Fairly shortsighted on them, considering they had either placed a weapon dedicated to planeticide on the hands of their champion, or at least observed its existence.