RewriteRule ^/img/(.+)\.jpe?g$ /img/$1.webp [L]
RewriteRule ^/img/(.+)\.png$ /img/$1.webp [L]
In .htaccess files, the URL-path that the RewriteRule pattern matches against, does not start with a slash. So the above directives will never match the requested URL, so won't do anything. In your earlier rules you are matching against a digit as the first character, not a slash.
For example, it should be:
RewriteRule ^img/(.+)\.jpe?g$ /img/$1.webp [L]
However...
In img folder in prestashop, there is another .htaccess
Depending on the directives contained in /img/.htaccess file then these directives might be completely overriding the mod_rewrite directives in the parent .htaccess file. The mod_rewrite directives in the parent .htaccess might not even be getting processed.
UPDATE: Having now seen the /img/.htaccess file then it's OK - these directives will not over the directives in the parent .htaccess file with regards to rewriting the request to the .webp images.
Since these additional rules only apply to the /img subdirectory and you already have a .htaccess file in the /img subdirectory then you could write these rules in the /img/.htaccess file instead (slight adjustment required). For example:
# /img/.htaccess
RewriteEngine On
RewriteRule ^(.+)\.jpe?g$ $1.webp [L]
The RewriteRule pattern matches against the URL-path relative to the directory that contains the .htaccess file. So, in this case, it does not match against the /img subdirectory.
To complete the rule and incorporate the other conditions you've mentioned you can do the following. Now back in the root .htaccess file:
# /.htaccess
# Rewrite image request to ".webp" if supported and exists
RewriteCond %{HTTP_ACCEPT} image/webp
RewriteCond %{DOCUMENT_ROOT}/$1.webp -f
RewriteRule ^(img/.+)\.(jpe?g|png)$ $1.webp [L,T=image/webp]
If the corresponding .webp image is guaranteed to exist then you don't necessarily need the filesystem check (the second condition).
(I'm not sure why you would need E=REQUEST_image?)
And if using the .htaccess file in the /img subdirectory:
# /img/.htaccess
# Rewrite image request to ".webp" if supported and exists
RewriteCond %{HTTP_ACCEPT} image/webp
RewriteCond %{DOCUMENT_ROOT}/img/$1.webp -f
RewriteRule ^(.+)\.(jpe?g|png)$ $1.webp [L,T=image/webp]
Aside:
RewriteRule ^([0-9])(\-[_a-zA-Z0-9-]*)?(-[0-9]+)?/.+\.jpg$ %{ENV:REWRITEBASE}img/p/$1/$1$2$3.webp [L]
RewriteRule ^([0-9])(\-[_a-zA-Z0-9-]*)?(-[0-9]+)?/.+\.webp$ %{ENV:REWRITEBASE}img/p/$1/$1$2$3.webp [L]
RewriteRule ^([0-9])(\-[_a-zA-Z0-9-]*)?(-[0-9]+)?/.+\.jpg$ %{ENV:REWRITEBASE}img/p/$1/$1$2$3.jpg [L]
The 3rd rule isn't doing anything and should be removed. And the first two rules can be easily combined using regex alternation on the file extension. eg. (jpg|webp).
