To what extent would a 17th-century observant Jew purposefully violate Halacha to save lives?

Question

I’m writing an alternate-history story set in the 1630s and this topic touches on one of the plot points. My character is attempting to avert the various massacres of the 17th–20th centuries by engaging more strongly with the Christian community. I am receiving a great deal of resistance from Jewish readers who tell me that the character is violating halacha the way I’ve written the story, and that he would not realistically do what I am suggesting he might.

To what extent is it permitted to violate halacha to save lives? What are the limits on this, whether in the immediacy of the threat (my character is particularly trying to avert the Holocaust, four centuries in his future) or in what actions are permitted (for example, attending, but not participating in, non-Jewish religious services)?

Would any of this be affected by the appearance of a society (in this story-universe, a town of 21st-century Americans) that values religious freedom and is appalled by any form of religious pogrom or sanctioned massacre? (I.e., would the 17th century Jewish community be more likely to adapt their practices, or double down on status quo?)

Answer 1

Answer to the question in the title:

Yes. To directly save a life, a Jew is allowed to violate Halacha. In fact, according to some opinions, a Jew is supposed to violate Halacha to directly save a life, even if a non-violating method is available, in order to emphasize the sanctity of life and the extent to which saving a life should be expedited.

Violating Halacha for indirect saving of lives, such as preventative action to avoid loss of life, would be permitted, but it would be greatly preferred that one employ a method which does not involve violating Halacha.

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Edit due to modified question:

attending, but not participating in, non-Jewish religious services

is generally prohibited. Only if your character could demonstrate that there is a clear and present danger which could be only be neutralized by attending a Church service would he be permitted to attend.

trying to avert the Holocaust, four centuries in his future

is not a clear & present danger, and all the laws regarding Saving a Life would not apply. In other words, a Jew cannot violate Halacha to possibly save lives hundreds of years in the future.

appearance of a society that values religious freedom ... to adapt their practices, or double down on status quo

A society that values religious freedom - ie, people can worship whatever and however they like - would not be accepted by a traditional Jewish community in the 17th century. (Or the 18th, or the 19th, and even the 20th in certain communities.) At best, it would be treated as any other Gentile society, and the whole concept of religious freedom would be ignored, and life in the Jewish community would continue as usual. If the idea of religious freedom started being accepted by the youth, and they started deviating from Tradition, the community would "double down on the status quo" and attempt to limit the influence of these "corrupting concepts."

Similar scenarios actually occurred in history, and historical Jews actually reacted to them. For more information, I suggest you research Jews in the Napoleonic era (and their reaction to the Jewish emancipation), and Jews in early America.