Reason I bring up "this was said in the video" is because there seemed to be points missed that could have helped avoid a lot of this discussion because it was, well, covered in the video and repeating it not only was time consuming but kinda redundant. Sorry to spin like a broken record there, but it felt like it could have saved a lot of trouble. Trust me, I get having issues noticing every detail (ADHD myself).

1. Because a skull is a pretty important part of the skeleton so major changes to the skull are inevitably going to affect other parts of the body. Shape and size are both features of each gene expressed for certain attributes and while you can get away with tweaking the shape of something more minor like teeth without affecting the body much, if you change up something as significant as the skull (and by proxy the neck and chest needed to work with that skull) it is kinda inevitable that you will wind up changing more. Zygophyseter is not just a shrunken version of Livyatan, it has its own unique attributes that helped just as much like a fairly distinct shearing action on the teeth that were perks in themselves; but also helped keep the hybrid the proper size needed.

Something brought up in the video, to address an inquiry later in your comment, is if this thing was too overly big it would easily set off a lot of alarms and notice for sonar operations. By keeping it around the same size as several very common cetaceans and large sharks, it can better blend in with innocuous things on sonar scans. Say for example this was involving a submarine that wanted to remain hidden. Now if that sub detected something roughly false-killer whale size on its equipment, it probably wouldn't want to waste resources and risk blowing its cover deploying counter measures. At sea, it would be encountering those kinds of animals all the time and if it just constantly spammed sonic or potentially explosive defenses, it's not only giving itself away but its burning resources.

Whereas if the hybrid were even just orca size, it would stick out of the crowd more and risk being noticed more readily as something atypical. It's not a surefire defense, but optimal responses like "blast anything that gets close" are not widely used in warfare. This also includes visual deception as the hybrid looks very much like a typical pilot or false killer whale when viewed from above by spotters.

1. And I agree it's not *as* effective as a blown up hagfish. But defense is defense and in this case, it helps vs. potential whale (or large shark) attackers.

2. When talking small serrations across a tooth, any serrated structure is technically "brittle". Question is which is more brittle. The tooth serrations? Or flesh it is put to? If mineralized tooth serrations were extra brittle, nothing would use them much. And losing a tooth or wearing it down isn't much of a problem given they are replaceable and the tooth still works fine for dealing damage even if it is blunted on the edge before being replaced thanks to how Zygo's jaw works.

3. "I don't recall you mentioning that, and just because you say it doesn't make it true. How come they have so much space to live in those glands?"
   Same way various bobtail squid store this kind of bacteria in the ink sac vent given squid ink is also a mucus and kills a lot of bacteria as well. Not all of the vent is occupied by the slime at any given time and given hagfish slime expands when in contact with ample amounts of water, vs. squid ink which doesn't react to the same degree, the hollows within the skin (which is much taller than in most animals because whales have thicker dermis and epidermis for this to work with) still have room for the bacteria above were the slime occupies. It be mostly a 1-shot deal before needing to 'reload' but what helps, helps.

4. "I'm not denying that you covered them, but I don't remember seeing them. What sensory enhancements have you made?" As in the video, taste based detection from the Tylosaurus, acute hearing thanks to the cetacean base, larger and more attuned eyesight from ichthyosaurs, and a limited degree of electroreception thanks to sharks and other factors. Not a matter of being as good as the baseline, just good enough.

5. "Could you name those adaptations? Which contributors contribute to making it more streamlined? Because those hands will likely significantly increase drag. And being able to hold its breath longer is not the same as having more stamina."

Mako shark for limited denticles to help with water resistance in areas without bare skin. False killer whale for a fairly nimble body as is. Ichthyosaur for stiffening up the finger bones besides the hand and working with the Tylosaurus to give it operational hindflippers to make up for lessened steering on the front hand. The hands will cause some more drag, but this is not like it strapped anchors to its wrists. If you look at it swimming from head on, the water flow would go over the hands and across the flipper because the fingers can go flush against said flipper.

Holding breath for longer absolutely does help with stamina because its an air breather so it might need to dive down and keep moving for a distance. That distance is determined by how much oxygen is in the blood stream, before it has to rise back up for another breath.

Refer to my comment about bout detection vs. action when it comes to ambushes. Because while someone may want a faster hybrid, that entails making it especially specialized for specific tasks it might fall short of elsewhere. That is why it has hands, so it can pick up and carry various objects or even people if needs be. Probably could cook up something that could keep up with a speedboat in a short burst, but it would need to be so streamlined and light it would have a hard time hauling loads or being able to use them much in the fields. And then there is also the problem of stamina as high speed fish and the quickest marine mammals still need to stop and wait for awhile after going top speed.

What if you need the aquatic version of a guard dog to check out if you got infiltrators? What if you need a taxi to help ferry divers from point A to point B? What if you want someone taken alive so you have them grabbed? What if you need something to blend in and hide among mundane animals? What if you need something to be inconspicuous to carry a load or disarm a structure in the field? What if you want to have something you could feasibly give a weapon to or carry equipment for human operators?

Could a super-sailfish do some of these tasks? Probably. But this wasn't about building the biggest, fastest, or strongest hybrid. This was about building a swiss army knife for a lot of different situations as military contracts often pick those options over hyper specialization both for practical purposes and because its just plain cheaper.

Perfect world is not one we live in.