Yeah the costume data follows straight after the badge data, your offset was probably just… off… set… by a byte or two.
I've just uploaded my
notes from the 3DS on how costumes work for characters (also Fairy outfits).
You can see on the "Mask Testing" sheet how the four bytes of costume data for each character are derived. The costumes marked with 'x' are always unlocked whether specified in the mask or not. Of note is that the first two bytes controlled costumes available in the base 3DS games, byte 3 covered costumes added by DLC, byte 4 never actually got used as no character had enough costumes for that.
As with a lot of things here I'm assuming no changes to IDs/Bitmask values between the 3DS and Switch versions of the game — only the offsets for finding this data in the save. From everything I've seen this seems to be the case but do not blindly assume
The fairy outfit stuff comes in two parts in the file, right next to each other (the offsets of which I have no idea about on the Switch yet, but it should be easy enough to figure our from just doing a before/after from getting an outfit[1]).
The first section is a bitmask covering what outfits are actually unlocked.
The second section, straight after, covers the order in which thing were unlocked (as you can sort by "newest" when choosing outfits). The ID (column B) of each outfit is added to the start of the section bumping everything else along by one. The full 255 bytes are padded at the far end with 0xFF IDs.
A similar thing happens with fairy food. Once you've found the location of the fairy food quantities you'll also find straight after it a section for tracking your most recently obtained foods. IDs for those are in the Fairy Food sheet (assuming they haven't changed between 3DS and Switch)
In terms of
fairy data I've uploaded what I figured about the 3DS version too for good measure. Some further notes about specific values can be found in
Turtleman579's two posts here the latter of which was the basis of my own values about fairy outfits.
These notes are all very crude and some of it you probably already know, but I've been meaning to dump all the stuff I figured out and never publicly revealed for a while now rather than just holding onto it. And by this point you've probably done enough tweaking of the save file that this stuff isn't totally meaningless to you.
[1] Alternatively you should be able to look at what outfits you have unlocked right now in your copy, and put an 'x' in column J for only those outfits. Column K should then consist of a series of bytes to search for. Again, assuming they haven't collapsed the list to remove the [CRASH] and [AUTO REMOVE] entries. Which given how relatively lazy the port seems to have been I would rate as unlikely.