It was always positional asphyxiation - the spine/back concern was more to do with travel systems being used in place of a proper pram, which is another no-no (but not actually life threatening).
Also have a 12yo - so remember this coming out initially! The concern for a while was over "bag slings" - you may remember this if you have a 10yo although it went away very quickly once they were banned, and the focus went onto car seat time. Then a couple of years ago there was the Fisher Price Rock n Play scandal in America which was positional asphyxiation again.
These are two different concerns although they are often conflated. The breathing position is something that builds up over an unbroken stretch in any device which is too upright or can encourage too much of a chin on chest position. Car seats do unfortunately need to be this upright for crash safety. The lie-flat ones don't perform as well for safety and are therefore only recommended for premature babies or infants with breathing difficulty for whom the oxygen desaturation risk is much higher than the risk of being involved in a crash. This is not the majority of newborns, and certainly not healthy, full term newborns over 4 weeks old. They are not in danger with their breathing unless you're using the car seat for really extended periods, they are much more vulnerable to crash forces.
The risk is not that the baby will stop breathing. That's not the problem. The problem is that if you're trying to get air and your chin is on your chest, it's like breathing through a kinked straw - hardly any air can get in. Try it.
If you're a healthy adult, older child, etc or even a newborn lying totally unrestrained in a flat cot, then what would happen is if you're getting into breathing difficulty because of this extra effort needed to draw air in, you will instinctively move. You know how toddlers fall asleep in all sorts of ridiculous positions, and it isn't a problem. They move if their body signals that they are becoming uncomfortable or something like blood/oxygen flow is restricted. The problem with car seats is that they have to be in a pretty tightly restricted position in order that the car seat can do its job properly in a crash. That means that babies and toddlers can't necessarily reposition themselves if they are in a position which isn't ideal. That can lead to them being stuck in a position where they're not able to draw in enough oxygen. In rare cases, that happens for long enough that oxygen desaturation occurs. It's not instant, it happens over time. A breathing monitor is not really much use here because to stop breathing due to a reduced oxygen saturation level is so far past the point of danger you aren't likely to be able to do much. An oxygen saturation meter would be a more useful monitoring device, but these aren't easily available for home use I don't think. There's Owlet, but it's not cheap. But in any case, it isn't really required because as long as you make sure to interrupt any long stretches in the car seat, your baby won't ever get to a dangerous level of oxygen desaturation, even if they do get into a position where their airway is compromised. They only need to be out of the seat for a couple of minutes in order to readjust position.
Different countries set the guidance differently. UK goes with total time limit in the seat counted from when they go into the seat to when you take them out, regardless of whether it's in the car, on the pushchair, on the floor, being carried. Other countries where people routinely travel longer distances by car emphasise the importance of not using the car seat as a sleeping space outside of a car. Either approach works as they both help you avoid the same scenario - the scenario (usually in a carry type seat) is that the parents drive somewhere and when they arrive, the baby is asleep so they decide to leave the baby in the seat. Or perhaps they run a short errand and the baby sleeps/is in the seat the whole time, and then when they arrive home the baby is asleep, so they decide to leave them in the seat. This time in the car seat all adds up and can cause the baby to cumulatively be in the seat for several hours. If you look at the cases where babies have died or got into difficulty it tends to either be a scenario like this, or it's where the seat has been used as a regular nap spot or overnight sleeping spot. In fact, the fixed seats that stay in the car are less of a problem because you tend not to leave them in the seat unless you're actually driving. Maybe occasionally to finish a nap after a drive. But not nearly as often as you do with a carry seat (I've had both!)
Concern about spine development is not car seat specific but is more about babies being moved between objects which seat them in an unnatural position and cause them to be restricted from moving naturally - they need some time to stretch out and move their bodies in an unrestrained way, e.g. in arms, on the floor, in a playpen/cot, and later sit normally rather than being forced into a sitting position by something like a Bumbo. A one off long journey won't matter for things like that, it's more day to day habits.