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If you are going to edit scripts, then I recommend using the Debug mod as that makes finding error messages a bit more reliable. There are supposed to be log files anyways, but some crashes skip creating a log. The caveat is that the Debug mod will attempt to keep going even if a lot of things break, which can corrupt saves if you ignore them and save your game.

There are the obvious ways to break a script like wrong data or forgetting a comma or bracket. But there are also the less obvious ways like using 4 spaces instead of a tab or a carriage return instead of a newline. The error messages are usually helpful in pinning down what was wrong.

Item data has its own unique quirks and unstated formatting, which is made less obvious because they are in dynamic data structures rather than a class objects. Due to many cases of basically redundant data storage in items, you can indeed alter a lot of item data without needing to match the script. However, in some cases the item data must match the script because of dictionary lookups or similar code, like how some person attributes require using specific values.

Stackable items(rope, potions, and such) tend to be heavily restricted by the script and unstackable items(gear) tend to be less restricted. Equipping items behaves similar to adding traits, so it's best to unequip them before altering them. Not all "effectvalue" in gear effects are used, some are hard coded in the combat script. Some data fields, like "type" and "effect" must be paired correctly or the game could easily ignore it or crash. Otherwise there should be no problems with tweaking or exchanging effects between items, or adding enchantment effects.

As a sidenote, I recommend getting the Bugfix mod as the script for at least one of the item effects was wrong.