Are Catholics free to disagree about whether it is prudential to not forbid abortion by law?

I believe the United States Conference of Catholic Bishops has anticipated your question to help Catholics to think about public policies and to vote accordingly within the United States political system. I can see how other current Western democracy-like political systems like those of Canada, Britain, and most European countries are covered by the principles espoused by this 2015 document called Forming Consciences for Faithful Citizenship. The 42-page PDF version can be downloaded here.

Some quotes [bold is mine, to highlight portions relevant to this question]

[From "Introduction"]

7. In this statement, we bishops do not intend to tell Catholics for whom or against whom to vote. Our purpose is to help Catholics form their consciences in accordance with God’s truth. We recognize that the responsibility to make choices in political life rests with each individual in light of a properly formed conscience, and that participation goes well beyond casting a vote in a particular election.

[From "How Does the Church Help the Catholic Faithful to Speak About Political and Social Questons? - Making Moral Choices"]

32. Sometimes morally flawed laws already exist. In this situation, the process of framing legislation to protect life is subject to prudential judgment and “the art of the possible.” At times this process may restore justice only partially or gradually. For example, St. John Paul II taught that when a government official who fully opposes abortion cannot succeed in completely overturning a pro-abortion law, he or she may work to improve protection for unborn human life, “limiting the harm done by such a law” and lessening its negative impact as much as possible (Evangelium Vitae, no. 73). Such incremental improvements in the law are acceptable as steps toward the full restoration of justice. However, Catholics must never abandon the moral requirement to seek full protection for all human life from the moment of conception until natural death

33. Prudential judgment is also needed in applying moral principles to specific policy choices in areas such as armed conflict, housing, health care, immigration, and others. This does not mean that all choices are equally valid, or that our guidance and that of other Church leaders is just another political opinion or policy preference among many others. Rather, we urge Catholics to listen carefully to the Church’s teachers when we apply Catholic social teaching to specific proposals and situations. The judgments and recommendations that we make as bishops on such specific issues do not carry the same moral authority as statements of universal moral teachings. Nevertheless, the Church’s guidance on these matters is an essential resource for Catholics as they determine whether their own moral judgments are consistent with the Gospel and with Catholic teaching.

34. Catholics often face difficult choices about how to vote. This is why it is so important to vote according to a well-formed conscience that perceives the proper relationship among moral goods. A Catholic cannot vote for a candidate who favors a policy promoting an intrinsically evil act, such as abortion, euthanasia, assisted suicide, deliberately subjecting workers or the poor to subhuman living conditions, redefining marriage in ways that violate its essential meaning, or racist behavior, if the voter’s intent is to support that position. In such cases, a Catholic would be guilty of formal cooperation in grave evil. At the same time, a voter should not use a candidate’s opposition to an intrinsic evil to justify indifference or inattentiveness to other important moral issues involving human life and dignity.

35. There may be times when a Catholic who rejects a candidate’s unacceptable position even on policies promoting an intrinsically evil act may reasonably decide to vote for that candidate for other morally grave reasons. Voting in this way would be permissible only for truly grave moral reasons, not to advance narrow interests or partisan preferences or to ignore a fundamental moral evil.

36. When all candidates hold a position that promotes an intrinsically evil act, the conscientious voter faces a dilemma. The voter may decide to take the extraordinary step of not voting for any candidate or, after careful deliberation, may decide to vote for the candidate deemed less likely to advance such a morally flawed position and more likely to pursue other authentic human goods.

37. In making these decisions, it is essential for Catholics to be guided by a well-formed conscience that recognizes that all issues do not carry the same moral weight and that the moral obligation to oppose policies promoting intrinsically evil acts has a special claim on our consciences and our actions. These decisions should take into account a candidate’s commitments, character, integrity, and ability to influence a given issue. In the end, this is a decision to be made by each Catholic guided by a conscience formed by Catholic moral teaching.