The government routinely compels speech. It's weird that we'd be trying to derive this from first principles: you can just go to your pantry and pick up any packaged food item.
It's an interesting example, food nutrition labeling. A couple of my initial reactions are that
(1) Courts initially make a decision about whether or not something constitutes "speech" versus a part of a business transaction or something like that. It's possible that a nutritional label isn't considered speech as much as it is product or a commercial transactional component or something. That doesn't mean the same arguments would or would not apply to a warrant canary.
(2) The food labeling analogy is generally forced transparency. So for example, the alternatives are to either not report nutritional information or to lie about it. The former is a legitimate concern I think from a free speech perspective but refusing to report contents might be reasonably argued to drift into the latter alternative, which is basically fraud. The government coerced speech is to move speech away from fraud.
Asking someone to continue posting a warrant canary basically amounts to coercing fraudulent speech, which is kind of tricky. It would be akin to the government compelling you to not only put nutritional labels on your food, but also to put false nutritional labels on your food. The government coercing continued posting of a warrant canary when a warrant has been served moves speech toward fraud, which seems really different to me from the case with nutritional labels.
rsync has posted here on the thread that they'd have to consult lawyers and whatnot about whether or not to continue posting the canary, and that might take time. I think this is more nontrivial than some people seem to give them credit for, in the sense that lying about it might induce various lawsuits about fraud from customers and stakeholders that have a a financial material interest in them being transparent.
I'm playing devil's advocate here and not trying to adopt any particular position. I think it's entirely possible a court could throw out the idea of the warrant canary on trivial grounds. But it seems to me coerced fraud is different in subtle but important ways from coerced silence.
Your (1) has a term of art: "commercial speech". Warrant canaries, at least of the sort we're talking about, are commercial speech --- even the EFF canary site implies it agrees with this.
"Commercial speech", the term you're looking for, is any speech that has any nexus at all to commerce. Warning labels are commercial speech (that's why the government can compel them), as, I'd presume, are anti-warning-labels.
There may very well be reasons warrant canaries will hold up in court, but "canaries aren't commercial speech" seems very unlikely to be one of them.
