1) See above^ pcs kill laptops, but they are not portable unless you build it in a mini-itx case, and that even then is akin to lugging an Xbox/PS with you, and you need a wall plug to power it wherever you go.
2) They each have their merits, but I will say that A) (this is partially opinion) No apple computer is worth it ever. I've seen them sell $3000+ laptops that have standard tech in them, they sell a $4000 "professional" tower that hasn't seen a major update in the last couple of years regardless of the fact that new tech has come out, they sell iMacs that, yes have 5k screens, but also run off of laptop graphics cards when you'll be using it essentially as a tower. B) iOS and Windows from what I've seen is more a matter of 'How much do you see?', where windows will ask where you want to install things, iOS will normally save it to your desktop or another predetermined spot. They also hide a lot of their files which are only accessible through their command prompt, but it's files that 95% of the population will never look at. Otherwise, they pretty much have the same operating basis. C) In terms of what she wants to do, a windows machine (which, as stated above, you will get better parts for than you will in an apple machine), will do the exact same tasks likely at a higher pace than a mac will. Contrary to popular belief, there isn't actual evidence that a mac is "better" for graphical designs. The only thing it holds over windows is that certain file types or fonts aren't able to accessed on a windows machine, so it's typically required in a business to have a few to handle every order coming through.
I've never used linux, however I think the "concern that she can't run the software she wants" is an absolutely legitimate concern. That would require a bit more research as to exactly what software wants to be run, but 99% of software runs on Windows.
3) Yes it is absolutely possible to get a non-touch screen laptop, I have one. There are a ton, so no worries there. A touchscreen laptop most of the time is essentially a glorified tablet, where they'll probably have an APU instead of a CPU and dedicated GPU, the APU or CPU/GPU will not be as powerful as they would on a standard laptop. That being said, you get the OS you want, and you get the bonus of tactility. That can be worked around through something like Wacom or a company like them that sells tablets that USB connect to your machine and allow you to have a drawing surface to work on, for a relatively cheaper cost than having a touchscreen laptop. They are quite expensive for the specs that they run.
4) That entirely depends on what you want to go with. Apple? expect $1500+ if you want a good machine. For that exact same price point on a Windows machine, you can get a killer machine, especially if it is a desktop. $1500 will nearly max out your components. If you're looking for a budget machine, plenty of PCs still run around $6-800 and can run very very well. My desktop is currently 4 years old running strong. Cost $900 at time of build. Right now, a similar (if not better) build would cost between $5-600. Maybe up to $650.
Gaming laptops typically run cheaper than a workstation laptop, because gaming GPUs are cheaper than a workstation GPU, my k2100M laptop GPU costs $5-600 by itself. If you up that price by maybe $3-400 you can get a fairly good laptop regardless if it's gaming or just a regular laptop.
5) The only other consideration I could see would be the above ^ to have her build it with you or someone else, especially if she likes tech. I would not be majoring in computer and systems engineering if my parents hadn't built PCs with me to show me this type of stuff.
Another consideration I would have would be, if she only wants the laptop for portability, to see if you can reformat the hard disks on it and put windows on it or put linux back on it. If it's not showing any signs of hardware failure, only system slowing, that could help it out. That would also give her the freedom of still having a laptop if she chooses to go with a tower.
edit: Another thought: (if she's really into the art thing) a higher res screen. 1080p is great and all, but 2k is even better. And you can find 2k screens that aren't all that expensive. For a price point, I built my roommate a machine earlier this year that runs any game that is thrown at it on high or ultra at 60 FPS on a 2k screen for $1300. Price of the 2k screen included.
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