What is stopping the parasite from morphing into more parasite cells, and using them to take over the entire body?
Two things. First, parasites seem to have a finite amount of mass. After healing Shinichi, Migi stated he lost 30% of his total body mass which is quite a lot. Trying to spread himself thinner could cause the parasite to lose itself completely as the smaller it gets the less IQ it possess. The second reason for Migi never targeting Shinichi's head; Once a parasite matures, it can only really stay as the body part it's set as. A parasite set as an arm or leg can't control the entire body because it's not the center of the nervous system like a brain. Likewise, Migi had to distribute cells all over Shinichi's body to form his new heart rather than directly making it out of his own form. The only reason he was able to temporarily act as a replacement was probably from all the biology books he read.
Why are parasites so resistant to bullets when they are shot in their human areas? The one that was in the school took a huge number of bullets to the torso before it died. Tamura Reiko also gets shot over and over again before she falls.
Ordinary bullets (such as those used by the police against Shimada and Tamura) only work against the parasites in large quantities because each bullet deals relatively little damage to the human body under the parasite's control. As the parasite feels no pain when its human body gets hit, it can keep on going longer than a normal human. This is why the authorities eventually develop shotguns with special shells against the parasites.
In episode 20, one of the parasites kills a bunch of armored cops. He approaches one of them and notes that he survived longer because he is wearing body armor, and he also picks up his gun, then throws it away. Why didn't that parasite take the armor and gun for himself? The main weaknesses of the parasites are 1) gunshots to their body(admittedly not such a problem to this particular parasite, but it couldn't hurt), and 2) the fact that their attacks are limited in range. A gun would eliminate that weakness. It just seems like a case of the parasite being stupid for no good reason.
The parasite in question is Gotou, the most powerful and durable parasite in the series. He simply does not need armor or firearms to be incredibly lethal and dangerous, as well as nigh-invulnerable. In any case, the shotgun would eventually run out of ammo.
There is probably also an element of pride involved. Gotou is proud of his natural abilities and would disdain being dependent on human tools. He also seems to enjoy testing his abilities, and a full-on assault by heavily-armed soldiers is a good opportunity.
Is it common in Japanese schools to have faculty meetings over the paternity of pregnant teachers, like the one they had in Episode 4?
While Japan is generally seen as a overall more technological society than the United States in such ways as public transportation and technology, they are not as socially progressive. Aspects of Japanese culture are farther behind than American culture in several ways by at least a few decades, like LGBT recognition and Autism support. While I am not a hundred percent sure on this, it does seem plausible.
Another issue is that Tamura was single and unmarried at the time. In the minds of her colleagues, at least, there was the theoretical possibility that she might have been having an affair with either one of the male teachers or one of the male students.
Don't forget that this manga also was originally done during the 1980s, even the Western world wasn't all that happy about an unmarried, single woman being pregnant. It's likely that they simply think that a single mother wouldn't do well in raising a child; and Japan also has some heavy emphasis on gender-roles. While it's okay for a woman to work, it's still expected in most cases for her to stop working and become a housewife after marriage (and pregnancy).
A visibly pregnant single teacher in Japan would be a huge embarrassment to the morals of the school, and doubly so in The '80s. Whether or not her career per se was in danger, the faculty would want to get involved and ensure that the school (as an organization) wasn't shamed. Whether that means her resigning, or taking a leave of absence until the baby is safely born and "out-of-sight, out-of-mind," would depend on the school and her existing reputation.
So I've been following this show on Toonami, and in a recently-aired episode, Migi and Joe(Jaw) talk about the uselessness of names and how parasites like themselves don't understand why humans are so into naming things. Parasites have ways of identifying each other from a distance, so they wouldn't need them, and they can morph so they don't need to describe individuals to each other. Humans, on the other hand, have no such long-distance signifiers nor can they morph, and so humans would need names. The parasites know these, so why do they question it? It should be obvious to them why humans like naming things.
Also, if parasites who have taken over humans feel compelled to eat other humans, wouldn't they become incredibly heavy?
I'd argue that all that tentacle action takes a lot of calories to do, so that's what happens to the human they ingest. They probably can also handle any chemicals that negatively impact them from eating humans.
Why doesn't Shinichi simply show Kana his powers? He's not trying to keep it a secret from her, he flat out tells her that he's got a parasyte and by this point in the story he is clearly super human even when Migi is sleeping. Nothing we're shown suggests that they live in a high enough power setting that Shiniji's powers are within the realms of plausible so him simply making a fifteen foot leap onto a roof (or however high the sea wall was) followed by a few more inhuman feats to at least make an honest attempt at putting her off. It would be different if he was still trying to keep her in the dark but he's not trying at that point.
Here's a Voodoo Shark for you: Nobody parses Shinichi's superhuman feats as a sign of him being inhuman. Satomi doesn't clue that anything's up when he jumps over a wall, she's too busy worrying about him having changed and become cold. Same with everyone else. So it wouldn't have been too useful as proof from that standpoint.
Why aren't there more parasyte animals? We saw that parasytes can posess dogs, so I would assume that every animal is a possibility. I would expect at least some of them to become big forest animals (bears, boars, sharks, whatever) and just live happily in nature as aphex predators.
The parasites specifically targeted humans and humans only. There are probably more animal parasites (especially when the parasites landed in lowly populated area like forests or oceans) but they're the minority.
Shinichi never once tells Satomi that his mother has died, despite her increasing concern that something is wrong, and telling her would provide a reasonable explanation for his emotional issues. Furthermore, once the police level with Shinichi and his father about the Parasytes, they no longer attempt to cover up Nobuko's status as missing and legally declare her dead, but no other cast member ever mentions it.
