Mass outbreaks: sure, if you modify the correct block. Which block is the correct block? Nobody has identified which blocks are responsible for mass outbreaks, or what is needed to be changed in each to get specific results. The block editor can do savefile diffs to let you know what changed between save files, so that will help narrow down the list to identify which is responsible. Remember that the save file changes can't make the game behave differently (like spawning unavailable species in the wild); that requires editing the ROM.
How likely a shiny would appear: no, outside of giving yourself a Shiny Charm. Changing the amount of shiny rolls the game does is a game-code change, not a savefile change. Use a cheat code to change the game code while the game is running: https://gbatemp.net/threads/pokemon-scarlet-violet-cheat-database.621563/