GRF Editor. Install it. Select your GRF files, ItemInfo.lub files, where is your pre-re folder located, and try to use it, with GUI it's much easier to edit / add something new. Just need to try (always do backups first). Try to play with it. It's easy. Optionally you can try to use something like this:
https://herc.ws/wiki/Custom_Items
As far as I know - nope yet.
No, it's not done. On your screenshot there is errors with applying the patch.
Most easier way to apply a patch - just open the patch with VSCode editor, it will highlite green text that need to paste, and red text that need to find in the file. Just use search feature in your text editor and replace. But there must be some understanding what are you doing and for what.
Better idea: tell me what do you want to do?
answer above. Also, as far as I know - patches from different emulators not compatible. Also -> emulator source code changed frequently. And if you're for example found a patch on herc board below for example 2018 year, there are high chances that this patch require changes to make it work and make it compatible with the emulator current source code. (because code changes overtime).
Also, hercules has some benefits over rathena:
Not need to download and install 8GB of Visual Studio for re-compiling the emulator.
