Nope, it's not totally laid flat. You'd still want either a carrycot, or a buggy where the backrest can be totally lowered to a flat surface. It's just flatter than traditional car seats which allows for better oxygen flow while in the car/on the buggy. The i-level goes to 157 degrees, rather than 180 degrees. If you measure from the other side that means a 23 degree angle, as opposed to the 30 degrees a typical car seat is when sat on the floor, or 40 degrees when it's installed in the car, ideal angle for crash safety about 45 degrees. Your car will be slightly steeper than 23 degrees, because rear car seats are not usually parallell to the ground, they slope backwards.
Guidance for carrycots/sleep spaces for babies over longer periods, just to compare, is no steeper than a 10 degree incline, and there is no car seat on the market which lies that flat. Only the carrycot type, which are not recommended in terms of crash safety. That's why there are time limits for car seat use - it is safe for them to be inclined, just not over an extended period of time.
I can't tell you about the headrest because it seems to depend on the baby - some of them don't mind it and get right to the end of use of this seat (over a year) but some of them seem to really be bothered by it and the parents want to move them on as soon as possible. I have no idea what the percentage is either way - it really seems to be a marmite car seat with some proclaiming that they love it and it was the best thing they got and others saying they hated it (heavy, inflexible, tight) and wouldn't recommend it at all.
Because it's a heavier/bulkier car seat, if you're planning to use it with a pushchair not made by Joie directly try contacting Joie to ensure it's definitely compatible. It isn't with all because of the physical size of the seat when it fits into the frame, but Joie customer service are pretty knowledgeable about this.
It is worth noting that the lie flat feature is thought to offer less protection in a crash than the upright position, so the upright position is recommended as soon as the child can cope with it.
I would also say to be aware that the recommendation to keep the baby in the car seat no longer than 30 minutes at a time is only for the first four weeks; after this and particularly if they were full term and have no breathing issues etc it's considered safe for them to be at an incline in a car seat for up to 2 hours at a stretch. Since you'll probably want to stop every couple of hours anyway on a long journey, personally I never found this to be a huge problem for travelling, and we do some really long drives (to Germany and back). Four weeks really isn't very long and you probably won't want to go anywhere at that age anyway. If you are concerned about the risk in general, I would recommend to do some reading/research about positional asphyxiation which is what you're aiming to avoid when you look at a lie-flat car seat. It's also been in the news due to the Fisher Price sleeper which was recalled recently in the US. The car seat research is also done using an extremely basic, and to be perfectly honest, outdated style of car seat, with no newborn support or flattening cushions which a great many of them have these days. It's good in a way because it means that if you follow guidelines your baby should be safe no matter what car seat they are in, but I do think it likely means if you have any modern mid-to-high-end car seat with a newborn support and some kind of flattening wedge or cushion for the younger babies, you can use some common sense, and relax a little over the guidance especially once your baby is older than 4 weeks.
I'd also say to look at the following seats which have a different system of flattening, just for some other options:
Cybex Cloud Z - similar system although can't be used in the car, only outside of it. Goes on a rotating base, which is nice.
Britax Babysafe 2 i-size with Flex Base - can adjust angle within car seat and on the base itself.
Nuna Arra - similar idea to Joie i-level.
Venicci Ultralite - keeps the baby's back in a flat position rather than curved, although the angle is still high - this is a good compromise, IMO. It's a rebranded Avionaut Ultralite, which is popular in Europe and the safety is good.
In addition, when looking at seats look for the inclusion of a "newborn wedge" which implies the seat has some kind of curve-flattening for a newborn. You don't necessarily need the full lie-flat system. For example the padding in Maxi-Cosi and BeSafe seats are very good. And if you have very sloped vehicle seats, the Joie i-Gemm with i-Base Advance is supposed to be a good combination.
Of these, the Cybex, Nuna, Venicci, Maxi-Cosi and Besafe use the same pushchair adapters (maxi-cosi type) as the Joie seats. The Britax uses Click&Go adapters which are compatible with Britax pushchairs and then any brand which is popular in the US tends to offer click & go as an option, but that does mean they don't go on some brands, like Joie and Cosatto for example, won't take a Britax car seat even though they can take the maxi-cosi type.