I think you’re a bit confused on how double effect works and how it would apply to abortion.
Let’s say there is a woman with an ectopic pregnancy that is almost sure to kill her if the pregnancy were to continue (the actual statistics show that around 50% of ectopic pregnancies spontaneously miscarry on their own, while overwhelming majority of the other 50% would presumeably be lethal).
The Ends: The intent in treating this condition would be saving the life of the mother. This is good, and the intent passes the standard.
The Means: This is where we examine the option of abortative procedures. The question is whether the termination of this innocent life is instrumental or incidental. Individuals are never morally permitted to use the termination of life instrumentally to achieve our ends.
But this is exactly what abortative procedures in this case entail. Consider: the physician performs the abortion procedure to dissolve the embryo, but the procedure is ultimately unsuccessful, leaving it to continue developing. Would we be frustrated or relieved at this? It is quite clear we would be frustrated, and this tells us that we are trying to use the termination of this embryo’s life instrumentally. Catholic moral theology condemns this.
A common proposal given by Catholic ethicists in this issue is the surgical removal of the section of uterine tube wherein the embryo has embedded. Consider: the section of tubing with the embryo is removed, and through unforeseen circumstances (this would virtually require a miracle), the embryo is actually able to survive to infancy. Would we be frustrated by this? Clearly not, and this shows that in this option the termination of the embryo’s life is not used instrumentally, but it is an incidental effect of our procedure to save the mother.
Consideration of the Effects vs. Ends: The final question we must ask is whether our ends are commensurate with the evil effects. That is to say, if the evil effects of our action are greater than our good end, we cannot go through with the action morally. In this case, the evil effects (the certain death of the embryo) are not greater than our good end (the life of the mother), therefore our course of removing the tube is morally permissible.
To return to the central point: in such a case where double effect is invoked, abortion is rejected by moral theology because any course of action that uses it necessarily invokes an instrumental termination of innocent life.