There is little or no RAW for in-depth injuries in D&D 5E, neither causing them nor healing them. Characters hurt in battle from hit point loss are assumed to be "worn down" in a generic fashion. There are a couple of special states (unconscious and dying), which also avoid dealing with the nature of wounds.
I think your ruling that a lethal attack, that would ordinarily have killed a character, can be changed to something equivalent depending on attack type (e.g. a sword or axe could sever a limb), is reasonable if used occasionally. Likewise, an emergency healing action that you would have ruled saved a life if done quickly enough, although not RAW, works OK for NPCs that would normally die when hitting 0 hit points.
I'm worried that I may have unknowingly opened a can of worms with
this ruling, and am wondering if there is any official guidance on
this?
If these events are kept rare, and are a replacement for a result that would kill by RAW, then I don't think it is a major problem. I am not aware of any official rulings, beyond effects of high level items in other editions, such as vorpal swords.
The 7th level spell Regenerate is the official fix for severed body parts - oddly higher level than Raise Dead, because in most D&D versions, being maimed is treated as a worse and rarer effect than being killed. You can take the existence of the spell as guidance that major characters can be maimed, and for how strong an effect the game wants you to treat it as.
Additionally, I am wondering if enemies should be similarly permitted
to 'attack to maim' the PCs?
Definitely not as a standard attack in combat. You might allow it to happen instead of a killing blow. My advice then would be to give the player the choice out of character, what happens to the PC. E.g. "That result would ordinarily kill your character outright, but if you want I could rule that the PC is unconscious with a serious long-term injury, which would you prefer?" - if the injury is chosen, you should work with the player to decide whether the PC will look for a cure or wear their injuries with pride. Also, pick something that can be worked around depending on the character's class (a one-armed archer character, whilst perhaps a heroically tragic figure in a story, may not be everyone's idea of "fun to play", and would definitely be sub-optimal in terms of numbers in combat).
Use your character's backgrounds to build intrinsic motivation for your players' characters.
If you're looking at the LMoP adventure, you hopefully have the pre-built characters handy. If you look at their backgrounds, you can see that for every character, there is some kind of intrinsic motivation built into their backgrounds for at least one of the elements of the adventure.
This provides the characters with clear motivation for why they should care about both the mission and the results of the mission.
You can do this in a custom setting in a couple of different ways. First, you could assign different background elements to your players, this may be a bit of a deal breaker for some players, but I've had good luck with it using a pre-built adventure and giving my players the option of which of 10 or so to select (Horde of the Dragon Queen provides 10 different optional background elements).
However, the best thing you can do is to pick up plot elements from your characters' backgrounds. Have them write a short paragraph or even just be super detailed when they are building their BIFT part of their background so that you can mine that for details and NPCs and other motivations.
If you're looking to provide intrinsic motivation, having it worked into your characters' backgrounds is the best and easiest way to do it.
Best Answer
D&D 5e has abstracted this away into Lifestyle Expenses:
(from Player's Basic Rules, p52)
You can describe this as including the payments on a house loan, if they want to buy rather than rent. And if they decide to sell up and move, tell them the money they made on selling the old place goes towards paying for the new place.