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4) But if double effect cannot be used, on what other basis could the use of abortive vaccines be justified (ie.  You can’t just say “remote material cooperation,” which is the same as “indirect voluntary,” because indirect voluntary must always pass the double effect test).

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The people who say receiving the COVID vaccine is not a sin do not need to justify anything. Someone who claims an act is sinful has the burden of proof. Moral theologians teach this, as I have said before. In the case of potentially grave sin, the proof must be morally certain, so that if there is any real doubt at all as to whether something is mortally sinful, theologians say one cannot be held to grave sin in committing that act.
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I'm not sure why you keep talking about the "double effect test". A double effect means two effects. Which two effects are we talking about with someone receiving a flu shot? I can only see one effect of that action -- the act of getting a flu shot inoculates him against the flu. What's the other effect?

Does Fr. Scott say in the original article that parents must definitely refuse to allow vaccination or that they must protest and make their objections known, iow, accept vaccination under protest?  

When the law was changed in New York State for the 2019-2020 school year, enough Protestant parents in the school where I taught for nearly 30 years withdrew their children so as to effectively close the school.  A number of similar “Christian” schools in the New York City area closed their doors rather than cooperate with this unjust requirement.  Sad to say, I know of no Catholic schools, either novus ordo or traditional, who did not acquiesce. 

 One family went to unusual lengths to keep their seven children in the school.  The paternal grandfather lived in Israel where vaccines are not required. The family went for a visit and the three youngest children conveniently came down with the measles.  

In New York itself, measles was making the rounds in the Orthodox and Hasidic communities.  Gov. Cuomo and Mayor DiBlasio’s attempts to legally reign in non-compliant religious schools met mostly with failure.  The Jews and Protestants at least put up a protest.

But an utero child is alive, while the cells taken from a fetal body aren't alive, or not for long.  Fetal cells taken from a dead child can ONLY survive if combined with something living (i.e. a virus).  Everyone will admit that fetal cells have to be taken from a LIVE abortion victim, so it stands to reason that such cannot survive without a life-force.  It's arguable that the virus provides such a life-force.  
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Yes, a dividing of of the cells would mean (to me) a cloning.  But i'm not a scientist, I don't know.

Viruses by their nature are not alive- they are inert. They have no nucleus and only inject genetic material(RNA) by docking on and injecting into a living cell. They must use the living cell to duplicate.