We were not. There may have been some in our government attempting to get them on our side, but Delvin would not have accepted any alliance with the Monsters against Enweil, and would have employed all his considerable political capital and rhetorical skill in attempting to prevent it. Nor was he alone in this sentiment.
If it had come to a showdown, it probably would have ended with an earlier Meridian Republic secession than ended up happening, but Riombara would not have allied with the Monsters against humans.
Alone, you are not Riombara. As you said, some of your government was attempting it. Had things played out differently, such as if he would have managed to have you removed somehow, it is quite likely that Riombara as a realm would have chosen this path. Pro-monsters and non-pro-monsters were to Riombara what monarchists and republicans were to RoF: conflicting opinions, but still the same people. Riombara and Meridian Republic were two sides of the same coin. Further, no matter your official policies, Lefanis was able to obtain some favors and concessions from the monsters that benefited Riombara, both before and after the secession. You opposed it, you prevented the realm from going fully with the monsters, but you failed to completely prevent it too.
You also weren't making any efforts to have peace with Enweil either. You were refusing Enweil's peace and military offers. So while you didn't want to look bad by siding with the monsters against Enweil, you also had no intent to help. In a way, you engineered a quicker fall for Rio so that it may have an excuse not to fight when Enweil needed it. Enweil didn't like Riombara, we did want you to die, but we also knew that our best odds against the monsters were if everyone fought them at the same time, to attack from all fronts in order to reclaim monster lands as soon as they were taken, so we put our lust for vengeance on hold and offered cooperation, which was rejected all along the SE's fight.