Shamim's Plugins

1. You mean in email? You can go to Dashboard > Front End PM PRO > Settings > Emails and set which content will be sent to email.
2. No

Attachment is attached to message by message id. So before sending message we can not know message id.
Why you need upload attachment before sending message? Attachment check (size, amount etc) is done before sending message. So if that check fail that will automatically fail message sending.

Also it seems you are not using latest version of this plugin. It is always recommended to use latest version.

You cannot have multiple function as same. You can change function name OR use anonymous function.
Add following code instead

add_filter( 'fep_form_fields',function( $fields ){
    unset( $fields['message_title']['minlength'] );
    return $fields;
});

Please follow https://www.shamimsplugins.com/docs/front-end-pm/customization/remove-minlength-message-title/

I have logged it for consideration.

instead of this shortcode use <a class="fep-button" href="http://YOUR_MESSAGE_PAGE_URL/?fepaction=newmessage&fep_mr_to=1,2">Message</a> (change YOUR_MESSAGE_PAGE_URL with your message page url and 1,2 with your actual user ids)

Message can be sent only one group at a time.

I have updated above code. Please use this code.
Then add following CSS in your theme’s style.css

.fep-column-cus_count {
    width: 50px;
}
.fep-column-cus_date {
    width: 100px;
}