The advice is inconsistent because the risk is small, essentially there are two risks to keeping them in their car seat too long. One is positional asphyxia which is very rare but also very serious. This is more likely to happen when they are younger, if they are asleep, and if their car seat isn't very supportive - many of the newer ones are much better than old fashioned ones used to be. Essentially if your baby's head, neck and spine are in a nice straight line then their breathing is unlikely to be impaired by their seat, although it's also not ideal for them to be at an angle rather than laid flat.
The risk of positional asphyxia increases with how long they are in the seat with around an hour being the point where it increases enough to want to try to interrupt their time in the seat ideally, although don't worry too much if you do sometimes go over the hour mark, because as said the chance is very rare. Most cases of car seat asphyxia happen when the car seat is habitually used as a nap location for the baby, ie every day as a routine rather than the baby happened to fall asleep in the car and you let them finish the nap in the seat. And it is common for babies to fall asleep in the car. Although it isn't ideal for them to spend a lot of time sleeping in a car seat, it's unlikely to be an issue if it happens sometimes. (OTOH if your baby will stay asleep, best practice is to transfer them to a flat location like a cot or pram bassinet when you get out of the car).
Up to an hour is usually no issue at all unless they are very small, have breathing issues or the seat is not at all supportive. At 10 weeks you're past the point where you would need to worry about stretches of less than an hour, but you might want to keep in mind the hour mark as a rough guide for wanting to take them out for a stretch.
Official advice used to be 2 hours, which shows you that it's not terribly dangerous to exceed the hour at times, it's more of a guideline. TBH, with young babies, they tend to not especially like being contained in a seat for multiple hours - they want feeding, changing, cuddling more often than that anyway. So IME, it is not at all difficult to use a car seat safely with positional asphyxia guidance in mind, even if you're using it on a pushchair for a day out etc. Alternatively, you could use the carrycot or pushchair seat for your pram and when you arrive where you're going, swap him out of the car seat into one of these seats rather than use the car seat on the pram wheels. We tended to use the car seat if we were going to the supermarket or doing lots of in and out of the car to avoid too many transitions in and out of different seats.
The other risk to babies with car seat use is that because their position in the seat is quite restrained, they are missing out on opportunities that they would naturally use to develop their muscles and reflexes for rolling, sitting, balancing and so on - which they would be able to do in a carrycot, on flat ground, or in your arms. When travel systems first started to become popular about 25 years ago, this was the main concern (called "container syndrome") as manufacturers popularised the idea that the car seat could be used as a multipurpose seat, for the car, the buggy, as a rocker at home and also as a feeding chair (when babies used to be weaned onto very smooth purees before they could sit up!) Some people were literally keeping their baby in containers like a car seat or a bouncer/rocker for most of the day every day. This is not good for babies as they do need lots of time with freedom of movement in order to develop, but obviously the occasional day that they spend more contained is not a concern.
So to avoid positional asphyxia = take them out of the seat for a break every 1-2 hours and consider moving them to a flat surface to finish a nap if they fall asleep in the car.
To avoid container syndrome = minimise time spent in the car seat compared with time where they can move around more freely.
Obviously when you are in a moving vehicle, the risk of a car accident outweighs either of these risks so it's important to keep them in the car seat. But on long journeys take a break every couple of hours so they can stretch and to protect their breathing.