the filepath for metadata is assets\minecraft\optifine\cit\vumo.properties - change the matchItems tag in the second line of the file to whatever item(s) you want, provided it's formatted by their in-game value (ex Carved Pumpkin -> carved_pumpkin). you can do this for any .properties file in any generic text editor, like notepad - ignore any warnings that might pop up from editing an unknown filetype, the data reads fine.

i personally havent changed the pack format at all but you probably can - it shouldn't cause any issues!

this is unfortunately an issue with the game itself - blocks lose any tag data like names when placed. hence they sit in item frames - you can use a datapack to make the frame invisible and decorate that way, though its not perfect

replaces carved pumpkins and totems of undying. i'll add it to the description, but it was also in the readme provided