So many unexplained terms and systems!!!

For that matter, we need numbers. How much does a healing potion heal? A high potion? How can we tell whichever to buy if wee don't know relative power?

Is there a point to having more than one of each weapon type, or should we just sell as soon as we get one with a higher ATK?

Coverage just means how big the attack is. How much space in front of Miriam the attack will cover.

ATK is a calculated based on the weapon's stats plus your STR stat.The ATK number is literally the amount of HP you deal with your standard attack (before other modifiers such as weakness and resistance are applied). Guns work a bit differently because the bullet type also matters.

DEF is calculated based on your current armor plus your CON stat.

INT is how much damage you deal with Shard attacks. Even "physical"-style Shards (such as True Arrow) still rely on INT. This is a lot less concrete than the ATK stat because different shards have different power, so some experimenting may be required to find which shards you like best. 

MND is your defence against magical attacks. It may also help with MP regeneration, though I don't know for certain.

LCK makes items and shard drops more likely, and I think it might also make critical hits more likely too.

A Potion heals 50hp. a High Potion heals 250hp. You can figure this out quite easily by using them and seeing the green number than appears above Miriam's head upon unpausing.

Eating food once gives permanent stat buffs. Food items are still useful as they can be consumed to restore HP. You can only hold a small number of potions, so food is a way of being able to hold a lot more healing items than normal.

Inventory in this game is unlimited, so there is no downside to keeping all of your old weapons. Weapons are often used in alchemy to create new weapons, or to upgrade shards, so don't get rid of them unless you are sure you don't need them for any crafting recipes.

I'll admit I can be a little critical, but there are some things in this game that are essentially random (how can I determine which stat it's better to raise if I don't know what stats do?) and some that seems wrong (I leveled up and my across the board weapon damage went down?).

And some of this is vagueness that make questions of efficiency harder, ie is the Large healing potion a better deal at 5 times the cost of a regular one or not? I can't check in the shop with full health, so it would be nice if the shop gave me enough info to know the answer instead of vague wording.

And there is some confusing wording. For example, the in game text never tells you food heals you, just that the first time you eat it it gives you a stats boost. So in battle you are gonna favor not dying by using the thing that explicitly says it heals, not the one that says it gives you a one-time stat boost and nothing else, making it seems like anything past the first item is useless. So based STRICTLY ON WHAT THIS GAME TELLS YOU, you think potions are the only source of healing and food is just a one time stat boost and nothing else. And if you are an old player the wording my lead you to believe that this has changed until you try it, which a new player may not think to do at all.

"It works the same as all the old games" is useless to a player who either a) never played them, b) played them long enough ago to forget (that's me) or c) is viewing this as a new game series and assuming nothing works the same until told otherwise. If you start a brand new game series, you have to assume no one knows ANYTHING you don't tell them in the game (or the manual if present) and have to treat everyone as complete and total newbs. Bioshock is a spiritual successor to System Shock 2 just like Bloodstained is a spiritual successor to Castlevania, but knowing how to play System Shock 2 won't tell you how to play Bioshock, nor should having played the prior decade+ old game be expected to be able to know how to play the new one. You will have new or forgetful players and they should not feel excluded or confused because they haven't played or don't remember everything about the prior games.

Yes, what Bloodstained's community really needs is to be kept for the True Igavania Fans, and made inscrutable to anyone outside that group! We must keep the bloodline pure! No outsiders! 

We certainly don't want to be exclusionary to outsiders, but keep in mind that the digital guide tier reward is in the pipeline, and that should explain most of the stats and all that Though now that I think about it, an in game help button might have been nice, yeah.

I guess I'm the only one reading the books?

The load screens giving explanations is actually a problem for PC users with SSDs. Loading is literally done in a second and a half which gives barely any time to read them, lol.