There are many questions I would like to have answered.

You can ask questions here but yes, it's a good idea.

What’s the rules for challenges? Can I buy the candies pack (or hack the game) to upgrade the helmet?

Also why do I have all birds unlocked from the start? It prevents me from running 100%.

https://discord.gg/nzjUtvUVnh

For Challenges category, you cannot use online features (like buying candies) or hack the game.

For the 100% category, you must start without save (you must begin with "Games played: 0" is shown).

But I’ve seen some people have an obscene amount of candy in the challege category and it makes sense because then you dont have to grind to upgrade the cave helmet before every run from 11-120.

Problem with the 100% category is that I have every bird, even when I redownload the game. I don’t even remember buying them.

For Challenges category, you can have any candies, gems and helmet updates you want. There are no rules about these items.

For your problem, you have to delete all data related to the game, but I can't help you about that as I'm on Android.