Frankly, I am speculating rather than working from textev. Yes, the unification has strong popular support. But it only takes a significant minority opposed to it to create lots of tension. We see that over and over in our own politics. If, say, a third of the people are opposed, Chava can use that to call into question the whole idea if he doesn't succeed in taking over from the Caliraths. That becomes even more true if the Arcanan threat which provides the primary stimulus for the unification recedes.
Surrendering sovereignty is a crash and burn step that is bound to have lots of people opposed to it simply because it's change.
As I read this post you appear to be talking about issues of legitimacy. I understand legitimacy to be the characteristic of a government which makes people obey its decisions without being compelled (no government can compel everyone all of the time) even when they disagree. If a government has legitimacy the opposition of a large portion of the people to a decision will not increase tension because the people who disagree will accept the decision because it was made by the legitimate government in accord with legitimate procedures.
You appear to me to be presuming that the decision to form an Empire of Sharona is not viewed as legitimate by the opposition. A legitimate government (and as far as I can tell the governments of Sharona appear to have had legitimacy when the series began) can lose its legitimacy in two ways. First, if it violates the procedures that legitimate its decisions. For example, the process of voting on a bill in the House and Senate and then having the President sign it legitimates laws in the United States. Second, if the decision is so offensive to the opposition that instead of accepting the decision because the government that made it is legitimate the opponents of the decision start to deny the legitimacy of the government.
In the absence of evidence that either of these apply on Sharona, and you said above that you don't have any, I am assuming that they do not. (Yes if a similar decision had been made on Earth the second would apply but Sharona isn't Earth and based on what we know of its history I don't think its nationalism is as militant as Earth's.) Thus while the reorganization involved in forming the new government will certainly disturb people I continue to doubt that the formation of the world government increases tensions in a meaningful or threatening way.