Luckily, I have pondered long and hard on the question, and can now triumphantly reveal the answer!
So, how can we explain Janet's failure to point out the many, many times that alien influences have manifested in bizarre behavior? (To quote Daniel in Season Seven, citing incidents that took place in Seasons One and Two: "Well, there was the time he got really old, the time he became a caveman, the time we all swapped bodies...") Why is Mackenzie so insistent upon his theories before Daniel is committed, yet is willing to listen to Daniel when he sounds just as irrational as he did at the beginning? Why does Jack only lamely offer a theory of "stress," instead of forcefully reminding Hammond of their past experiences? How come Teal'c doesn't volunteer to go back with his Junior-protected immune system and search for an answer?
Even when Daniel has recovered, the brain quota seems sadly under par. When Sam, Janet, and Jack are trapped in the iso room and suffering from multiple little buggers, why does an infected Janet have a greater grasp of medicine than a sane Warner? Why is Warner so pessimistic about their chances, even as Sam insists that they keep trying?
After careful, detailed analysis (i.e. watching the eps) of both Legacy and Holiday, the answer becomes clear: it's all Machello's fault!
Machello's behavior when he is in Daniel's body proves that he has little sympathy for any innocent bystanders who might be lost in his battle with the Goa'uld. The man was brilliant, and he thought in terms of failsafes - consider that little quirk of his body-swapping machine, which deliberately made it impossible for the same bodies to switch back. So it stands to reason that Machello built a failsafe into his little buggers, to wit: they infect all human bystanders with stupidity!
This explains everything! Now we have a reasonable explanation for why the Linvris locked themselves in - their servants (because c'mon, even if the Linvris didn't necessarily have Jaffa, no Goa'uld worth its dorsal spines would go around without servants) panicked from the stupidity fog and shot themselves, shooting so wildly that they disabled the lock on the door in the process! (There were no remains of the dead servants, because either they accidentally shot each other three times with their zats or the former Linvris hosts, er, managed to find some sustenance before dying of starvation after all. Er. Sorry.) Because really, why wouldn't the newly-released hosts have wandered out the door and gone to their former homes if they'd had a choice?
And now we can understand why everyone so complacently went along with the Terran-centric schizophrenia theory, instead of taking a few minutes to wonder if it might be a result of some kind of alien influence. We can now comprehend that Mackenzie is probably a decent, intelligent guy under normal circumstances - after all, the man does survive at least seven years at the SGC, and hidebound thinking would have left him dead (or dismissed) long before that! And as soon as the little bugger was out of range and Mackenzie could think straight again, he did listen to Daniel and call Jack. (aurora_novarum, by the way, will be posting her own meta on Mackenzie later this week, for a much more detailed analysis of the guy.)
This also explains Dr. Warner's bizarre behavior, when he repeatedly dismisses the possibility of isolating the protein marker, and the team is forced to depend on poor infected Janet to remember basic medical procedure - like everyone else in range of the stupidity infection, Warner's IQ has dropped by several dozen points. Luckily for SG-1 (and Janet), the buggers release an anti-stupidity marker into the blood of its victims, which is why Daniel and Sam were able to be calm and rational even in the most desperate crises.
Y'know, a lot of people use "PTD" as shorthand for the "page turning device" that actually contained Machello's little buggers. Little do they know that it actually stands for plot-hole transmitted disease!
See??? It all makes sense now! :)