[Question about the setting?]

Don't worry, there's plenty of nature-based stuff to explore once you get outside Reborn City! If you want you can look at the map inside your Pokegear and see what's in the city and what isn't, but if you'd rather not to avoid spoiling it for yourself that's understandable. In regards to the difficulty, you're completely right. This game is extremely challenging. However, allow me to say this: you ain't seen nothing yet. There are a ton of super tough fights that are nothing like what you've seen up to just the first 2 gyms that require strategizing, team swapping, and/or numerous attempts to win. Anyway good luck and have fun, I'm sure you'll have a great time!

[Shiny Pokémon]

1 hour ago, Gastronely said:

I believe the shiny rate is 700/65,535 (1.07%)

And with the Shiny Charm I think its like 3 times more

[Good teams for Shelly?]

You could also grind up to level 35 for everybody and that would probably make things easier

[Where to find houndour?]

It depends on which gang you join. If you join Magma Gang then you get one as a gift from Maxwell after defeating Aqua Gang, but if you join Aqua Gang then one appears in the overworld for you to catch after you defeat Magma Gang. Eventually you get to a point where you can catch them in the wild, but that's not until very far into the game (13 badges).

[Poisoned/Nightmare (move) Possible Bug]

Clefable has Magic Guard so it shouldn't be taking damage from poison or Nightmare in general. The rest of them I can't really speak for.

[jukebox question]

It seems to me like a slowed down version of Atmosphere- Mirage, but I can't see any track that plays it exactly.

[Pokemon Reborn Engine?]

It is Essentials as far as I've seen.

[Terra Battle (Screw Godchomp and stupid seeds]

29 minutes ago, Atg303 said:

I'd try that but I cant get back to the move relearner in reborn city because of the sleeping event in agate city (I believe). Any other suggestions?

The clown the farthest south and west in Agate Circus is another move relearner. The devs thought ahead when they locked you out of part of the game.

 

Edit: It's not a clown and it's in the southeast, but it's still a move relearner.

[Mewtwo in glitch city]

48 minutes ago, VividGrey said:

Start with Furfrou and use sucker punch to get that priority super effective damage.

 

When he's down, try going for crunch with Mawile maybe?

The problem with that strategy is that in the Glitch Field, Dark moves do Normal damage. The only thing that Psychic is weak to here is Bug, since the field also makes them immune to Ghost. In that case, the only super effective move you could get with your current would be to reteach your Sandslash Fury Cutter. A strat you might be able to run with that would be to lead with something that has Snow Warning like a Vulpix or Snover or what have you in order to utilize Slush Rush. If that isn't enough then you might have to either try and brute force it or train up something that can take it down.

[Yet Another "Please Rate This Team" Post]

Ohh tbh I never actually realized that the Growlithe egg trade was one of the encounters determined at the start of the game, although I guess it makes sense. But dang that kind of ruins some of my plans with future runs.

[Lost Exp Share (Somehow?)]

Technically you can get more Exp. Shares by getting extremely lucky at the Lottery Corner, but that's not very likely. I would say to double check your bag and if any of your Pokemon are still holding it, otherwise you might've accidentally thrown it out.

[Yet Another "Please Rate This Team" Post]

Unless it's your starter, you don't get a Froakie until extremely late in the game, and Torchic is almost as far up there. Beldum isn't available until after you get the 13th badge and Goomy is after the 14th or 15th. You have a pretty low chance to get a Togepi fairly early in the game which you can try to SR for, otherwise you can only get it around the same time as Goomy, and Grubbin is available really earlygame but you have to wait until you get the 7th badge before you can fully evolve it. 

If you're fine with waiting that long to have your full team then you should definitely have fillers that you're comfortable with, you would be keeping them for a while. If not then you may want to rethink some of them, or just try and get what Pokemon you can as they're available.