Thanks for the link to the paper. But I don't think that 10-20 seizures per prevented suicide estimate you mention can tell us anything about probable cause or the lack thereof. It's a guess at the rate of prevented deaths from suicide attempts, when the subject used a something other than a gun, but would have used a gun if the cops hadn't taken it from them. It's not an estimate of how many suicide attempts were prevented: it's just a measure of how much less deadly the suicide attempts that happened anyway were. A gun seizure is a big dramatic intervention; it's pretty plausible that it sometimes interrupts a bad moment that's essentially a one-off, and sometimes leads to people getting the help they need. Neither of those effects play into the estimate you cited.

Just on the legal standard: it's probable cause that there's a risk, not probable cause that the risky event will happen. If you have 51% certainty that the subject has a 25% chance of harming themselves or someone else, that is likely to be good enough. And while it gives me no joy to defend the US criminal justice system, to me that's appropriate here. Living with someone who's making violent threats towards you or themselves is no fun at all, even if there's only a 1 in 4 chance that they'll follow through.