Its Simple PC & NPC are all at MAX i.e. Level x(Hit Die + CON Bonus). The only creatures with less than one hit die HP are some of the rats.
D&D Rules are mere guildlines
Any book rule may be ignored becuase it seems strange or unrealistic or unworkable or becuase the GM doesn’t like it. House Rules and ignored book rules effect NPCs and Players
All adventurers have quick draw otherwise they would be dead.
If you have attack actions left (even if you aren’t at your initiative) you can elect prior to the attack roll to parry. For a successful parry you must beat your opponent’s attack role by 2 or more. There are penalties if your opponent has greater reach, larger weapons or is substantially stronger than you. Penalties are up to GM Discretion.
If you are about to die your god may smile upon you. GM secretly roles percentile dice and if it’s below your ½ character’s level plus or minus modifiers for your role playing of your devotion to your god, you live, just.
The GM may determine that due to your combined skill selection or circumstances or excellent role playing that you have for an encounter or part of a game session an additional skill bonus. NEVER LOOK THE GIFT HORSE IN THE MOUTH
Buying & Selling:
If you don’t role play buying and selling (the GM really doesn’t want to its boring) 45% of book price is standard to sell and Buying is at least book price if not higher. You can roll diplomacy to increase or decrease this amount and each 5 points of success add 5%. failing penalises 5% per 5 points of failure.
GM IS GOD
You can question or debate results, circumstances and the like but once a GM Ruling is made that ends all discussion on the topic.