How to test code that's a significant chunk into the game? (pokeemerald)

[Moistnar]

Just wondering how people test stuff that can't be tested at the beginning of the game--for example, I just added Lunos' Gen 6 EXP Share mechanic, and want to make sure that the way in which I resolved my merge conflicts didn't have unforeseen consequences--however, in Emerald you don't get the EXP Share until after the 1st gym. I also want to add a cutscene much later into the game, and when I do that I will have no clue how to test it without playing through the whole game to that point. Any advice is appreciated

 

[Lunos]

Just wondering how people test stuff that can't be tested at the beginning of the game--for example, I just added Lunos' Gen 6 EXP Share mechanic, and want to make sure that the way in which I resolved my merge conflicts didn't have unforeseen consequences--however, in Emerald you don't get the EXP Share until after the 1st gym. I also want to add a cutscene much later into the game, and when I do that I will have no clue how to test it without playing through the whole game to that point. Any advice is appreciated

You can implement debug menus into your project to give you items or modify the values of flags and vars at will.
Alternatively, you can do those things in a disposable script.
Before I added TheXaman's debug menu to my personal project, I used to do my tests with the sign in Littleroot Town or the one in Route 101.
warp, giveitem, setflag, setvar. Those scripting commands do the exact thing their label describes.

If you don't know about overworld scripting, you can either check the game's own scripts to get an idea, like for example, Littleroot Town's.
Alternatively, there's Avara's tutorial.