Separate artists/composer/etc. with ", " and "& "

[tastiness6140]

I have some fields with multi-values such as composer (%composer%), featuring artists (%featuring%), artist (%artist%), etc.
I want to merge a certain field with multi-values with a separators ", " and "& ". I want mp3tag to determine the number of entries to a field and append "& " on the last part of the entry.

For example:

Original: Jack Antonoff\\Lana Del Rey\\Judah Smith
New: Jack Antonoff, Lana Del Rey & Judah Smith

Original: Lana Del Rey\\Jon Batiste
New: Lana Del Rey & Jon Batiste

How can I achieve this? So far I can do basic action such as merging fields with a ", " separator.
I'd appreciate anyone who's willing to help. Thank you so much.

[ohrenkino]

First merge the fields with the separator ",".

Then apply an action of the type "Replace with regular expression"
Search string: (.*), (.*?)
Replace string: $1 & $2

Please note that if will be difficult to reverse that manipulation as there are artists with names like
Earth, Wind & Fire
Bell, Book & Candle
Blood, Sweat & Tears
Brooklyn, Bronx & Queens Band
Dave Dee, Dozy, Beaky, Mick & Tich
Emerson, Lake & Palmer
Kitty, Daisy & Lewis
Stock, Aitken & Waterman
Up, Bustle & Out