The only way to 'copy' between metadata entries is include_in_X.
This works for me, but I only tested a couple of cases in a clean config.
Given I know you have a considerable amount of other metadata manipulation, I would not be surprised if this had problems with something else. I have a vague recollection of empty author list entries behaving weirdly with some esoteric configuration.
Personally, I would use this while downloading those stories then remove it or comment it out.
Code:
# make a copy of title
add_to_extra_valid_entries:,titlebyauthor
include_in_titlebyauthor:title.NOREPL
# add that copy to author
include_in_author:author,titlebyauthor
add_to_replace_metadata:
# clear titlebyauthor when NOT that series
titlebyauthor=>^(.*)$=>&&seriesUrl!=https://archiveofourown.org/series/1618510
# rest only for that one series:
# Remove author entries that DON'T start with "
author=>^([^"].*)$=>&&seriesUrl==https://archiveofourown.org/series/1618510
# Remove '"title" by ' from remaining author
author=>^\"(.*)\" by (.*)$=>\2&&seriesUrl==https://archiveofourown.org/series/1618510
# Remove "s and author from title
title=>^\"(.*)\" by (.*)$=>\1&&seriesUrl==https://archiveofourown.org/series/1618510
The author from the title will still have the real authorUrl. I consider that a feature.