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Inspired by the work Charles Duffy did in this answer and the work of Jonathan Leffler in this one and because I couldn't leave well-enough alone.

I went and wrote expandTilde.sh:

#!/bin/bash
doExpand() {
 local path
 local -a resultPathElements
 for path in "$@"; do
 : "$path"
 case $path in
 "~+")
 path=$PWD
 ;;
 "~+"/*)
 path=$PWD/${path#"~+/"}
 ;;
 "~-")
 path=$OLDPWD
 ;;
 "~-"/*)
 path=$OLDPWD/${path#"~-/"}
 ;;
 "~")
 path=${HOME-~}
 ;;
 "~"/*)
 path=$HOME/${path#"~/"}
 ;;
 "~"[0-9]|"~"[+-][0-9])
 local num=${path#"~"}
 local op=${num%%[0-9]*}
 num=${num#[+-]}
 local opath=$path
 if [ "$op" = "-" ]; then
 ((num+=1))
 fi
 path=${DIRSTACK[@]: $op$num:1}
 : "${path:=$opath}"
 ;;
 "~"*)
 local username=${path%%/*}
 username=${username#"~"}
 IFS=: read -r _ _ _ _ _ homedir _ < <(getent passwd "$username")
 if [ "$homedir" ]; then
 if [[ $path = */* ]]; then
 path=${homedir}/${path#*/}
 else
 path=$homedir
 fi
 fi
 ;;
 esac
 resultPathElements+=( "$path" )
 done
 local result
 printf -v result '%s:' "${resultPathElements[@]}"
 printf '%s\n' "${result%:}"
}
expandAssign() {
 local -a pathElements
 IFS=: read -r -a pathElements <<<"1ドル"
 : "${pathElements[@]}"
 doExpand "${pathElements[@]}"
}
expandString() {
 doExpand "1ドル"
}

So two questions:

1. Did I miss any cases or get any cases wrong? (Tests in the github repo.)

2. Can this be improved in any meaningful ways?