You can definitely change it if you want, I don't think valve will remove scripts/console commands, even if they do you'll just have to relearn it at that point. It's definitely a bit weird and takes some time to get used to it but at the same time the attack key is useful not only for denying but also for attack moves, which lets you move to the targetted position or attack the first viable target in the way, whichever comes first. This is useful when chasing someone in the fog if you want to attack them as soon as they appear on your screen without requiring an additional click which might be too slow if they fog you again and such. Also useful for jungling with additional units like spiderlings, chen creeps, illusions and so on, attack moving on the map so you don't have to micro as much.
On courier and shop, mostly what you want to do is have a key for the courier(and one for your hero) so you can swap selection without moving your screen or your mouse, then buying depends on people, you can use shortcuts, you can type the name, you can click the menu, you can set a quick buy beforehand and so on. After that it's just mechanically pressing the buttons for take items(if you didn't have the courier selected when you bought), bring item, turbo. Rest is mostly automated anyway, courier goes back to base once it has delivered. For the shop it helps to setup stuff in advance say you want to buy a meka next you shift click it, it adds all the parts to your quickbuy menu for when you get the money, you can do that when you're healing in base or dead for example. You can only setup one full item at a time though.
I also recommend checking guides. In the top left corner of the screen, I think 2nd button from the left(first is to go into options), there's a guide button. It opens a list of guides for your hero, you choose one(generally the highest rated), click it and it'll change your recommended items list and stuff, as well as have some explanations. It also shows you which skill to train next with more explanations. It's a pretty useful feature to learn new heroes as the base recommended items are fairly awful.
As for pubs, you can play against bots in coop mode, assuming you get lucky you'll have an actual full team of real players and not just people afking for items and you can see how retarded people are by getting yelled at for playing bad against bots but mostly how people tend to move, the mistakes they do that bots don't and shit like that. After that though yeah it's real games and it'll most likely be awful for a while.