You can leave it rear facing 
The manual says that because it's illegal to use any car seat forward facing before your baby is 15 months old, as it's dangerous for them in a crash before this age. The forward facing mode becomes a legally allowed option from that age onwards.
Between 15 months and outgrowing the seat (roughly age 4, but depends on your child's height) you can choose whether rear facing or forward facing works for you. Rear facing is much safer, especially so the younger they are, which is why it's required by law for the youngest babies, so best to keep them RF for as long as it's working for you - I'd definitely aim for a minimum of 2-3 years if you can. The Joie seats don't have the best leg room but it's OK for their legs to be crossed/bent, not a safety issue for them
Growing with the baby isn't necessarily a sign a car seat is good/better - it's mainly a marketing thing. Though if your baby is approaching 15 months, then I agree that a 0-4 years one is a good option The other thing to adjust as they grow is to make sure that you're pulling the headrest up as she gets taller, so that the straps are at the right height for her or it will be difficult to fasten her into the seat safely. It is also more comfortable for your child when the headrest and straps are at the right height.
Essentially about 40 years ago when all the different car seat types were first invented, you had a reclined/carry type for young babies who couldn't sit up yet, and then a sitting up, forward facing type from about 9 months onwards. These days, because we know more about the safety benefits of rear facing, it's common to see combination type seats which can do both rear facing and forward facing from birth up to 4 years old. While you can use these seats from birth, most people prefer the carry type seats for when they are little, as they sleep so much and it's much easier just to be able to lift the seat in and out and carry them around without disturbing them. It is also usually easier to get a good fit for a newborn in one of the carry type seats because they have a much smaller total age range.
The carry type seats don't usually accommodate bigger children because it starts to get too heavy to carry them around and their legs hang out and it just isn't practical, basically.
While it's not ideal for babies to sleep in car seats, it's OK for them to do this sometimes and while they are very little and sleeping several times a day, it can be hard to avoid. You shouldn't have a car seat as your baby's main bed - they need a crib, Moses basket or cot - and it shouldn't be used as a nap space when you're at home, but if you've driven somewhere in the car and they are asleep in the seat, you can carry them inside and let them stay sleeping. It is a good idea to try and keep a rough eye on the time that you first put them into the car seat and then make sure they aren't in there for too long without a break. The guideline given for the time limits is 30 minutes for newborns up to about 4 weeks old, and then 2 hours for older babies and this just applies from the time you put them into the seat until you take them out, it doesn't matter if they are sleeping or awake.
For your next baby if you already have a carry type seat from your older child you could reuse this again. That is practical if it works with your pram, and it means you can keep your older child in the Joie seat until the baby is too big for the carry seat. You could then get a larger rear facing seat or forward facing seat for your older child depending on what you prefer at that time.
For travelling, sometimes people transfer their own car seat into the taxi, or sometimes they buy a second, lighter seat for travel, something like Graco Extend LX can be good. Or like with the grandparents, if you are going to visit somebody that you know, they might be able to arrange their own car seat or borrow one for the journey for example. Abroad sometimes you can hire them with a hire car, but these are sometimes of questionable condition and quality.
Hope that helps, happy to answer any more questions!