May a Relic Be Venerated Instead of a Crucifix?

Well, yes, as long as the relic is enshrined in a cross. But, frankly, it would be better just to have the crucifix for the liturgy and have the relic exposed during the day for veneration. I do know one parish that had a cross especially made to house the relic for use on Good Friday. This seems to me legitimate.

In the basilica of the Holy Cross in Rome, I myself have witnessed that on Good Friday they use the relic of the Holy Cross brought to Rome by St. Helena in the fourth century. So there is some precedent for using a relic for the adoration of the cross.