Hi, Isa, and welcome to the forum. I am afraid that your instincts are 100% correct and it IS irresponsible to sell unweaned baby birds because, as you have noted yourself, this is something that requires hands-on experience PLUS it's also not good for a baby bird to be by itself (meaning without siblings and/or parents). Unfortunately, and as much as they would argue about it, breeders don't really love birds - if they did, they would not use them as merchandise and selling young babies is better for them because they can put the parents to breed again and make more money out of the season (and that's only if they don't breed them all year round, as some of them do). But don't feel bad about making the mistake of buying a baby - we all made and continue to make mistakes when it comes to caring for our birds. The important thing is that you realized it and are now learning from it and trying to correct the situation. It's not the 'not making a mistake' that makes us good birdkeepers, it's the wisdom of admitting to it, learning from them and not repeating them.
Now, you will find many different opinions on this - they will talk about a certain number of hours in between, a certain number of CCs per feeding, etc, but, personally, I would feed the baby until the crop is full and do it every time it asks for it, leaving a single period of 6 hours during the night (I don't really have any true experience on feeding baby parrots because I am against buying babies but I have raised many wild baby birds and a couple of canaries successfully and that is the way I've always done it). The crop does not take 6 hours to empty, it takes much less than that -even in adult birds. Sheesh! adult birds take about 2 hours from feeding to pooping that same food so, obviously, it does not take 6 hours for a baby. Please make sure the baby does not go hungry because there are studies that tell us that birds that are under stress while still very young (and going hungry is not only very detrimental to their health is also terribly traumatic for a baby) remain high-strung for the rest of their lives.
As to weaning it, yes, eventually, the baby will reject the hand-feeding and what you need to do is to offer it two different kinds of soft food served warm and fresh twice a day (once in the am and once in the pm). What we, birdkeepers, call soft food is, basically, food that is the equivalent for birds of what we would feed a toddler: soft, nutritious and easy to digest. I always recommend gloop as one of the soft foods because that is what I recommend everybody feed their adult bird so it only makes sense that they start as babies and get used to it. But there are other things you can use.. things like polenta, whole grain cous cous, pastina, Irish oatmeal (you can also use the regular plain old-fashioned oatmeal but I find it too messy) cooked and mixed with fruits and/or veggies puree (as in the baby food jars). The baby should also have some sort of soft seed -like millet- all the time but this is not something you should continue once the baby is a juvenile. NO CHEERIOS!!!! No human cereal, as a matter of fact, because this are foods that were formulated for human consumption and the dietary needs of humans are not the same as birds. Especially in iron content of which humans need a lot and birds need VERY little. If you look at the nutritional label of Cheerios, you will see that most of the varieties offered have about 40% iron which is WAAAAYYY too high for a bird (they require a max of 2% and the excess -which cannot be eliminated naturally by the body- ends up in their livers and causes a fatal, incurable condition called hemochromatosis).
As to the temperature, I would use the heating pad and I will tell you why. When birds are sick, they need to be kept at a high temperature for their bodies to be able to heal themselves - and when I say a 'high temperature' I mean 90 - 95 degrees. I am not saying your baby is sick but all babies require a higher temperature than adults because a) their body mass is smaller and b) they cannot regulate their internal temperature as well (I am raising feral kittens right now and they have a heating pad in their basket 24/7 even though the temperature inside my house is a very comfortable 70 degrees). Parrot babies are born in cavities that do not open directly to the outside -which makes it easy for the environment to stay a constant, higher temperature- and mother birds lay on them to give them their body heat. Birds body temperature varies by species but it's between 101 and 105 degrees so, to them, 90 degrees is really not hot but very comfortable.
Let us know if there is anything else we can help you with -and tell us what the baby's name is - we all think of our birds as our babies