Those cells were the initial cause of the cell line (in all four senses of causality); what happens subsequently is irrelevant insofar as we are talking about the causal chain as a whole.

This seems too general to me.  If such is the moral logic we must use, then everyone who has ever shopped at Walmart (or a 100 other places) is guilty of sin, because they sell chinese products which are DIRECTLY made from communist forced labor camps, catholic persecutions, forced abortion social programs, etc, etc.  The initial cause of 99% of walmart's products is godless atheism, no question.

So by the term “fetal cells,” you exclude placental cells?

@Pax Vobis - inferring from your posts, your reasoning assumes that in order to apple double effect principle we need that "2-3 steps from an abortion". That somehow is supported that abortions needed to the cell line happened 40-50 years ago.
What if the abortions are constantly need as a supply for either new tests or just maintaining the cell line or scaling up necessary for mass production. Would this be the same situation?

I'm the first to admit that i'm not an expert at all.  I wish I knew more about the process to create a vaccine; then there would be more details to debate.  As it is, I/we are debating based on general knowledge and facts here and there.
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Of course, if abortions were "constantly" needed for vaccines, then their use would be 100% prohibited.  As it is, it appears that the process is using CLONES/COPIES of the cells, which originally came from abortion long ago.  To me, a copy is a new thing, even if it's cellular makeup is identical.  Since it's a new thing, then philosophically speaking, it's essence is not the same as the original fetal cells.
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Then, the 2nd part of my objection is (which I don't know the answer to), "Does the chemical process of combining fetal cells with the virus substantially destroy/assimilate the fetal cells, so they no longer exist, or they can no longer be identified, because they have become part of the new entity, the vaccine itself?"  We know that fetal cells have to be taken from a living baby, so it stands to reason that they cannot survive unless they are attached to something living (i.e. virus).  So does this attachment process, mean the fetal cells are absorbed by the virus?  It sounds plausible.
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But my arguments all presuppose a few conditions:
1.  That all vaccines have a similar process and abortive fetal cells in their origin.  In other words, there's no alternative non-abortion option.
2.  That such vaccines will somehow be forced on us, either directly or indirectly, and to avoid one would cause substantial harm.
3.  Under normal conditions, I would never get a vaccine, nor would I advise anyone to.  I'm just playing "devils advocate" regarding a near-future scenario where the "new normal" gives us all few options to avoid one.

"The three most reliable sources to date of abundant fetal stem cells are the placenta, amniotic fluid, and umbilical cord blood." Source: https://www.sciencedirect.com/topics/medicine-and-dentistry/fetal-stem-cell

Dr. Judith Mikovits "Plague" is a book to read about vaccines. The process is pretty much always contamination. More viruses.

It is not possible to discuss the one without reference to the other. In fact, the whole use of material cooperation hinges on it.

What Church teaching requires Catholics to be martyred?  Were Catholics under Henry VIII guilty of mortal sin when they swore their oath of loyalty to him as required by the Act/Oath of Supremacy? Most Catholics did not refuse.

Obviously they were guilty of mortal sin.

It was a formal act of apostasy which they were obliged to resist even unto martyrdom.

I get the distinct impression some here believe their religion ought not inconvenience them, much less ever require martyrdom.