Remove multiple slashes but preserve trailing one - .htaccess

Question

I have a rule in my .htaccess to remove multiple trailing slashes. It works:
RewriteCond %{THE_REQUEST} ^([A-Z]{3,9})\ (.*)//([^\ ]*)
RewriteRule ^ %2/%3 [R=301,L]
I also have a rule that adds one trailing slash if it's missing. On its own, it works, too:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !/$
RewriteRule ^(.*)$ http://%{HTTP_HOST}/$1/ [R=301,L]
What I'm trying to achieve here is pretty obvious - remove multiple slashes, leave one. But with first rule applied, I'm getting no trailing slash on index page even when I specify it.
Please help me to combine these rules into one or at least get them to work properly. I tried to use this tool for debug but it is unable to process %{THE_REQUEST}.
I'll leave rest of .htaccess file for the reference, maybe it interferes. It wasn't written my be but I have to figure it out.
RewriteCond %{REQUEST_URI} !^styles
RewriteCond %{REQUEST_URI} !^css
RewriteCond %{REQUEST_URI} !\.css
RewriteCond %{REQUEST_URI} !^js
RewriteCond %{REQUEST_URI} !\.js
RewriteCond %{REQUEST_URI} !^images
RewriteCond %{REQUEST_URI} !^uss-js
RewriteCond %{REQUEST_URI} !^uss-css
RewriteCond %{REQUEST_URI} !^uss-img
RewriteCond %{REQUEST_URI} !^uploadedFiles
RewriteCond %{REQUEST_URI} !(\.ico|\.ICO)$
RewriteCond %{REQUEST_URI} !(\.jpg|\.JPG)$
RewriteCond %{REQUEST_URI} !(\.jpeg|\.JPEG)$
RewriteCond %{REQUEST_URI} !(\.png|\.PNG)$
RewriteCond %{REQUEST_URI} !(\.gif|\.GIF)$
RewriteCond %{REQUEST_URI} !(\.css|\.CSS)$
RewriteCond %{REQUEST_URI} !(\.js|\.JS)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ index.php?reference=$1 [L]
RewriteCond %{REQUEST_URI} (\.ico|\.ICO)$
RewriteCond %{REQUEST_URI} (\.jpg|\.JPG)$
RewriteCond %{REQUEST_URI} (\.jpeg|\.JPEG)$
RewriteCond %{REQUEST_URI} (\.png|\.PNG)$
RewriteCond %{REQUEST_URI} (\.gif|\.GIF)$
RewriteCond %{REQUEST_URI} (\.css|\.CSS)$
RewriteCond %{REQUEST_URI} (\.js|\.JS)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ [R=404]

Answer 1

I'm getting no trailing slash on index page
Yes you won't because your trailing slash adding rule has this condition:
RewriteCond %{REQUEST_FILENAME} !-f
Which means if request is for a valid file then don't execute.
You can add a trailing slash in your first rule also:
# remove multiple slashes from URL and add a trailing one
RewriteCond %{THE_REQUEST} //
RewriteRule ^(.+?)/?$ /$1/ [R=302,L,NE]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule [^/]$ %{REQUEST_URI}/ [R=302,L,NE]