That's interesting. A quick note. It's probably useful to distinguish "secondary boycott" and "secondary strike" here. The cited material sometimes interchanges those, seemingly due to the fact that the former has historically encompassed the latter. It seems plausible to me that someone might have differing views of those two things so they probably ought to be distinguished.
I guess a lot of meaning rests on the use of "coercive" here. What immediately comes to my mind when I see that term is violent threats directed towards someone so it's worth noting that isn't what's being described here. Rather it's being used to describe the effect of a boycott (in this case the secondary one) on the targeted (third party) business. That seems tautological to me because what is the point of a boycott if not to nonviolently coerce the targeted business?
So the court seems to be saying that if you view businesses as individuals then it is illegal to organize with the aim of coercing one person not to associate with another. The indirection (going via the consumer) doesn't seem to be the issue here. Your second link cites an electrical workers case where picketing with the express goal that the employer should terminate a subcontractor was deemed illegal.
Certainly I would agree if the coercive measure was violent, but is that really the case if it is exclusionary in nature? Would the court find the same way if I told my neighbors that they must not hang out with some guy who lives down the street or else they won't be welcome at my pool parties anymore? I certainly don't think that's a good way to conduct oneself but it seems inconsistent that somehow upon adding economic activity into the mix my speech can suddenly be restricted in ways it couldn't before.
To me it seems to fundamentally be a matter of free speech - engaging with other people and managing to bring them around to your way of thinking on the matter. If you hadn't managed to convince a large group of consumers (or non-unionized laborers) of the merit of both your position and your proposed course of action then they wouldn't be participating in the first place. It seems to me that the court is ignoring the part where your ideas won out as well as ignoring the individual agency of the consumers (or other laborers) and treating this as nothing more than direct action on the part of the organizing union when it is clearly much more than that.
To your example, I don't see any issue with Phyllis advocating and would note that she isn't going to get very far (sans coercion) unless Chad's employees, and by implication the community at large, generally agree with her. That is, Chad has to be the odd one out for this to work. That's not a pretty situation but I certainly don't want the government hamstringing community organization either. Free speech and freedom of (non)association can't only apply when it's convenient for those currently in power.
To my mind the issue with your example is the ability of a public facing business to discriminate along class boundaries (ex homosexuals) as opposed to on a purely individual basis (ex person who previously shoplifted). That would be in keeping with how other public areas are treated.