Let’s have a look at unity and solidarity in the world.
If human’s body only made up eye which is the organ of sight. We could not taste the real life properly. So, instead of one organ, Allah Almighty created different organs such as foot, hand, head, nose, mouth, etc… and what we call life is appeared miraculously. But if the organ of sight says to other organs: “Come and act as what I do.” Such a statement is both harmful and impossible. Therefore what is the most appropriate statement for the all organs to say is “Our purpose should be one. We all should serve to maintain our body in accordance with our ability.”
Also, different professions of society must be regarded as the different department of an army like the ground, naval and air forces.
The differences of Islamic communities, tarikahs and schools may be considered like this as well. They are all like the parts of a body. But they should have the same goal; “To serve people for the sake of Allah”. The differentness of the names is not important if the aim is same.
In this case, one of the most important points is not to run down one another. For example if you ask someone; who is the perfect mother in the world? “Doubtlessly my mother” he says. He is right with that answer in his opinion. But this answer never means that there aren’t any other perfect mothers in the world.
Similarly, when a question like “which path or temperament is the truest?” is asked you. You have right to say that your path or temperament is the truest. And other groups have right to think that their path or temperament is the truest, either. But no one has right to say: “Only my path is true or my path is the only beautiful one.” If this rule of conduct is understood, the divisions and enmities among groups end up.
And this situation is acceptable for the communities as well. If we deal with this as we mentioned above, we hope there will not be any separations between communities. Because communities are the organs which maintain the spiritual life of the society. They will gain consent of Allah from such an assignment of duties if they all have the same goal.
So as a conclusion, we would like to express that, since every people have the different characters and there are many parts of the religion, there will always be different communities, sections and groups in Islam and so there will always be different opinions (ictihads) and ways. If these opinions and ways are not contrary to the essences of Islam, then all opinions are right and all ways lead the people to the right path and to the consent of Allah.
And for the unity of Islam we say that it is not an obligation to make all ways same, but all ways should be helpful to each other, that is to say, they should not bear a grudge and enmity against each other.
As for your question about how to assemble all the masaliks at one point for a perfect ijtihad;
Among nowadays scholars, Bediuzzam Said Nursi in the Risale-i Nur Collection says that:
“In order to have a right, substantial, objective and all-embracing ictihad, an assembly including an expert team must be constituted. And the experts taking place in this assembly have to be known by all communities as dependable, objective and away from stains. In this assembly there must be as well as the experts such as in the fields of Arabic, interpretation, fiqh (Islamic law), the study of the Qur’an (theology), hadith and etc… also the experts in the fields of what is called positive science such as medicine, astronomy, mathematics, sociology, psychology, history, geography, etc... An ictihad made by these experts coming together, will conduce to both all-embracing, long-lasting and the gathering of all ummahs (community).”