The impression I got is that the whole Mawhrin-Skel situation was planned from the start by the same Minds that came up with the whole plan. Presumably, they thought that Gurgeh would not agree to leave his home and play Azad for them without Mawhrin-Skel's threat. So their plan would look something like: Get a fully operational Special Circumstances drone to play the part of the disabled Mawhrin-Skel, set up the blackmail at an appropriate point, and then let Special Circumstances come in with their rather conveniently-timed offer. Once he accepts, Mawhrin-Skel gets a new body and takes on his new role as Flere-Imsaho to guide Gurgeh through the mission.
As to the other point, near the end of the book, Flere-Imsaho explicitly tells Gurgeh:
Coming in ‘all guns blazing’ as you put it is almost never the right approach; Azad—the game itself—had to be discredited. It was what had held the Empire together all these years—the linchpin; but that made it the most vulnerable point, too.
So, SC's goal was never a military conquest of the Empire of Azad - they could already do that anytime they wanted to. They wanted instead to discredit the game of Azad in the minds of the empire's elite to destabilize the empire, as that way, the Empire would fall on its own. In the introductory section, where the SC drone Worthil is briefing Gurgeh on the Azad mission, he says, with regards to direct intervention:
We might be forced into a high-profile intervention against the empire; it would hardly be war as such because we’re way ahead of them technologically, but we’d have to become an occupying force to control them, and that would mean a huge drain on our resources as well as morale; in the end such an adventure would almost certainly be seen as a mistake, no matter the popular enthusiasm for it at the time. The people of the empire would lose by uniting against us instead of the corrupt regime which controls them, so putting the clock back a century or two, and the Culture would lose by emulating those we despise; invaders, occupiers, hegemonists.
Essentially, they could destroy the Azadian military easily enough, but that would leave the people with a flag to rally around and an enemy to rally against. A violent, degrading, and resource-intensive occupation would be the only solution at that point. With or without military conquest, the elite of the empire are too unified for Contact to come in and play one against the other to achieve their goals of reshaping the society, and any direct war would only intensify that.
If instead they are able to discredit the lynchpin of the empire and let it collapse on its own, the threat dissolves and the situation becomes more fluid. SC can subtly aid the resulting factions that are most aligned towards their goals, and subtly hinder those most opposed, without ever making themselves the enemy.