[ownedbycats]

https://manual.calibre-ebook.com/template_lang.html

In particular, check out the various list functions. For #1, list_difference() can remove entries. Regexes are helpful here.

If it helps to see examples, here's an old version of my #subjects composite.

program:

tags = $tags;

	if 
		'^(Fiction|Nonfiction|Magazines & Publications)' in $#booktype 
	then 
## Split tags or return an empty
		split_tags = re(tags, '\.', ',') 
	else
		split_tags = ''
	fi;

## Removing a few unwanteds and sorting
	cleaned_tags = list_sort(list_difference(
		split_tags,
		'[Cleanup], Fiction, Nonfiction, Magazines & Publications, Cultures & Regions, Topics',
		','), 0, ',');

## Add 'Omnibus' if applicable
	if 
		'omnibus' 
	in 
		$#admintags 
	then 
		cleaned_tags  = 'Omnibus' & if cleaned_tags then ', ' & cleaned_tags fi
	fi;

	cleaned_tags

For #3, there really isn't a way to automatically count chapters and any method to do so would probably be fairly unreliable. I use 'Edit ToC' (a bit faster than opening the book), check there, and then manually input.