* Tuck is able to speak to Jack from anywhere in Jack's body - that's up to a good four or five feet from the receiver in Jack's ear. So how come Tuck can't pick up from or send to the receiver when he's in Lydia, but still within four feet?
** For that matter, why was Tuck in Jack's mouth for the kiss?
** There's an explanation to this in the film: the only time we see Tuck in Jack's lower half is after he was injected into Jack. After connecting the audio transmitter and the camera, the farthest down that Tuck's pod is ever seen going is the stomach. That suggests that there may be a distance inside Tuck has to maintain in order to send/receive. Between your inner ear and your stomach is roughly 18 inches (depending on your body habitus, basically your body shape and how your internal organs sit in said shape). So, that means that due to the scale difference, it would be roughly a mile or a mile and a half away from the ear for Tuck. This would explain why Tuck didn't go back to Jack's butt or down to his feet afterwards, because there's only a certain distance it may be only able receive in. Also, part of the thing to remember that there are layers of skin and muscle that separate the inside of a human being from outside, which could make it difficult, if not impossible to receive a signal. This also sort of explains why Tuck was in the area of the nose/throat of Jack (remember, the nose, mouth and ear are all connected to one another), so that he could stay within transmitter range and not worry about losing the signal, and just happned to be in the area of the mouth out of happenstance, that lead him to be inside Lydia for a little bit. This is why Jack can't hear Tuck (and vice-Versa) while he's in Lydia: because due to the scale, he's farther away from Jack by miles (in Tuck's scale). And before you answer, the difference between Jack and Lydia while they're literally side by side would be, at least, four feet (so Lydia would literally have to be at kissing distance for Jack to be able to hear Tuck (that is if he wasn't in Lydia's uterus and was in her ear like he had been when they kissed for the second time).
* Why did Jack's doctor, co-worker and boss get invited to Tuck and Lydia's wedding?
** Probably because in the interim, they became Tuck and Lydia's friends too?
** They needed more guests to fill out Tuck's side? Does Tuck strike you as the sort of guy that keeps close to his extended family.
** The Doylist answer, of course, is that he could finally complete his transformation into a proper badass in his own right by telling them all to go screw themselves in awesome fashion. As for a Watsonian explanation, the co-worker at least got caught up in the chaos surrounding their adventure at least once during the movie (the nightclub scene), so they might have extended her an invitation as a way of saying thank you for the help / sorry for the inconvenience. Alternatively, Tuck and Lydia might have allowed Jack to bring along a couple of plus-ones so that he'd have at least a few people he knew at the wedding and wouldn't be entirely surrounded by strangers, and since his social life pre-movie doesn't exactly have seemed to be on fire these might have been the few people he knew well enough to invite.
** Or to better serve an in-universe explanation: Those three are the only people he had in his life prior to the events of the film. There doesn't seem to be any hint that Jack has any family, let alone friends outside of work and his doctor. It's a bit of fridge horror, if not a tearjerker thing to realize that before Tuck and Lydia came into his life, his manager, a girl who worked with him he really liked that had no interest in him and his doctor were the only ones he had in his life as friends.
* So Tuck's vessel caught champagne that Jack drank, and Tuck didn't find it at all nasty or unhygienic to drink it himself? It contained Jack's saliva and bacteria!
** It's ''alcohol.'' Maybe not a medical-grade disinfectant but good enough for Tuck, an alcoholic in a desperate situation. I don't think he'll have too much of a problem with that.
** Also, his mission was to go into a bunny and then come back out. He probably originally figured he would have been out of the bunny by that time (hence he had the empty flask in his case), and seeing that now he was in a human being and most likely to die in a few hours if they didn't find the chip in time, what's a little saliva in good alcohol gonna hurt?
* What prevented an immune response the instant Tuck and Igoe entered Jacks body?
** Might be the materials both pods were made from. A person who gets a pacemaker can suffer what’s known as “a pacemaker infection”, which usually happens within the first 12 months. Seeing that we know Tuck’s pod and some of Igoe’s pod (pre-bailing out after the thruster was damaged) were made of metal and were in the body for such a short time, it probably wasn’t long enough for the body’s immune system to notice Tuck or Igoe.
* Why did the rival lab build a miniaturization chamber that required [[ContrivedCoincidence exactly the same kind of chips]], even when the rest of their technology seemed far more advanced? The miniaturization process in the film is depicted as breaking up the object into its component molecules and a sampling of those molecules is then collected and used to "compile" a microscopic version of that object. The loss of so much of the object's mass is presumably the reason why the administrator described the re-enlargement process as "tricky", and why two chips are used as they can serve as a guide for exactly how to rebuild the object.
* One of the scientists at the government lab recognizes Dr. Canker, implying she may have been hanging around gleaning blueprints and ideas for awhile. The chips are just too new and complicated for her to copy.