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).

Anything referred to as an "attack roll" is made against the AC of the target.

Whenever you make an attack roll, whether that's with a weapon or a spell, you are rolling against the Armour Class of the thing you're attacking. I don't have access to the cut-down rulebook that comes in the Lost Mine of Phandelver starter set so I cannot reference that or tell you exactly what the wording in there is, but in the Basic Rules we have these rules about making attack rolls (PHB pg 193-194):

Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

...

When you make an attack, your attack roll determines whether the attack hits or misses. To make an attack roll, roll a d20 and add the appropriate modifiers. If the total of the roll plus modifiers equals or exceeds the target's Armor Class (AC), the attack hits. The AC of a character is determined at character creation, whereas the AC of a monster is in its stat block.

This last part is somewhat dubious because there are lots of things that can change a character's Armour Class, like putting on or taking off armour, casting spells that affect AC, or gaining the benefit of certain magic items (options which are also often available to the monsters/NPCs that the characters will fight, too). I think the starter set you're playing probably avoids that as much as possible though to simplify character management for beginners.