Alright, I'll give it a go:
1.Passwords are a great way to customise your experience, they range from quality of life as: a different starter, not having to worry about IV's, even more difficulty, monotype run helps. I think realistically speaking, only you can decide what kind of experience you're looking for. Passwords are not mandatory by any means, and without using any you can have an amazing experience too. During the gameplay, you can add passwords you might want with a consumable item called Data Chip.
2. Okay, so whenever a pokemon is generated, it has it's own Internal Values, or IV's, they are static and normally can't be changed (There's a limited amount of items in post game that can though, but that's post game). Those values range from 0 to 31 in each stats and you can check them in the EV/IVs menu in summary. You can manipulate those values with breeding, but that's a whole another topic, EV's on the other hand are effort values, basically, whenever you defeat a pokemon, the pokemon you use will get like 1-2 points in a stat assigned to the defeated pokemon, let's say I defeat a machop, for every machop I defeat I am getting +1 EV in attack, Ev's cap at 255 in one stat and I believe at like 512 total I think. Evs are much easier to manipulate and you can decrease and increase them using items, or just raise them fighting pokemon.
3.it depends on a pokemon, but generally almost in every case, maxing out speed and attack/special attack will do the trick, unless you're going for a specific build and obviously maxing out speed on a base 25 speed pokemon will be a waste, I would recomend checking Smogon for some builds for pokemon that you're interested in.
4.There's forecasts on tv, as far as I know, weather can't be manipulated anymore, so check forecasts frequently.
5.I assume you mean about pokemon like teddiursa that runs away if you interact with it, those are event pokemon, they don't require pokesnax, sometimes though when conditions are met, you may find pokemon standing in seemingly random questions, if you have pokesnax, you will be prompted to give it to them, if you do, you have a chance to catch it, it's a good way to get some good pokemon early on.
6.Numel is a pokesnax pokemon that stands near Grandview station in peridot, during rain.
7. The best advice, is to have fun, there is no wrong way to play this game.