oh wow that's a lot to respond to, I'll do my best :)
1) the first light upgrade you get reveals traps to the right of your character. If you're moving right step by step you're mostly safe, but if you move in any other direction you may fall into a trap. When you die, you also get the chance of revealing the tile to the right of where you died.
2) most areas (an area is a set of consecutive rooms with similar names and background and music) have a chest revealing requirement: step on every tile, automatic, get to the end of the screen without dying, defeat the wolf, enter the room from the right side, touch all 4 flowers. A few treasures require you to exploit the "entering a room from the top or bottom" mechanics, which may erase some traps. The night vision goggles reveals every tile you step onto and to your right, which includes chests.
3) there's actually a big chance you reach heaven before having collected all regular chests, so the night vision goggles help you get the chests you miss if you backtrack. You can unlock it after grabbing every heaven chest and a few of the regular ones. Also as you've noted in 5, the arrow slopes are tiles and it helps a lot to have the goggles by the time you reach hell.
4) they're dogs, but dangerous. They always move opposite to where you are and reveal nearby traps, except they kill you on contact unlike dogs. Your 6th question answers this.
5) you reveal tiles to your right and tiles under you with night vision goggles. The GOD upgrade lets you reveal all 9 surrounding tiles.
6) The wolves and dogs only reveal traps, not other tiles like chests and slopes. However you are right about them being much better scouts than humans in any area that is not hell. Greatest companions 🐶
7) endings trigger only if you enter a room with one of the 3 passwords. The game will actually not judge you unprepared to enter hell at all and the section is possible to complete without any of the upgrades (some players on my discord server did it) but you might have a really bad time trying this. Your hubris has brought you there.
8) ending 2 is only available after you've discovered heaven, decided that you wanted to explore more anyway, circled back through hell, and came back to heaven to decide you want to stay there. There is also a secret ending in heaven for those who make the journey there without a password, and then press Escape and accept to stay there without venturing forward into the depths. You immediately have a choice between going through the depths or staying in heaven forever, but the game is enticing you to continue, because you're curiosity will never be quenched, because you must always know what's next, because you always want an ending.
Everyone may have a different journey, but the intended one is to reach heaven with a few upgrades, realize there is more to the game, either go back to grab all upgrades or continue immediately, venture into the depths, suffer into hell, get knowledge in the memories of the end, and then decide for yourself whether heaven is the right place for you after all. Ending 1 is for smartass players who think they can try a password in the password room without having been told any passwords.