May we have crosses in our churches?

To the best of my knowledge, this is a matter upon which Mrs. White did not comment. Arthur White said as much many years ago, and I have never found anything in Mrs. White’s writings to prove him wrong. However, here is a statement from The Great Controversy that may have a bearing on your question regarding the cross:

There is a striking similarity between the Church of Rome and the Jewish Church at the time of Christ’s first advent. While the Jews secretly trampled upon every principle of the law of God, they were outwardly rigorous in the observance of its precepts, loading it down with exactions and traditions that made obedience painful and burdensome. As the Jews professed to revere the law, so do Romanists claim to reverence the cross. They exalt the symbol of Christ’s sufferings, while in their lives they deny Him whom it represents.

Papists place crosses upon their churches, upon their altars, and upon their garments. Everywhere is seen the insignia of the cross. Everywhere it is outwardly honored and exalted. But the teachings of Christ are buried beneath a mass of senseless traditions, false interpretations, and rigorous exactions (568).

You will notice in this statement that Mrs. White saw irony, not condemnation, in the Jews’ regard for the law and the Catholics’ regard for the cross. Just as she did not condemn the law, so she did not condemn the cross; but she noted how ironic it was that those who revered these things did not live by their real significance.

In regard to decorations for our churches, Mrs. White did not condemn such things if they were not extravagant and added beauty or function. Motivation was also very important to her.