If you don't want to use twentytwelve_entry_meta(); in the post loops that calls the date, category and adds that "This post was posted in" text itself, then you'll have to remove it and re-write from scratch.
1A: Comments and Number displayed, refer to the comments_number function: http://codex.wordpress.org/Function_Reference/comments_number
<?php comments_number( 'No Comments', 'Comments: 1', 'Comments: %' ); ?>
1B: If this is coming from Jetpack, it will automatically place them for you. If you want to change where they appear, take a look at this topic: https://wordpress.org/support/topic/manually-inserting-share-buttons
2: To show the current category(ies) that your post is in, refer to the the_category function: http://codex.wordpress.org/Function_Reference/the_category
3: To change the format of the date, refer to the get_the_date function: http://codex.wordpress.org/Template_Tags/get_the_date
<?php echo get_the_date('G:i | F jS, Y'); ?>
As well as the PHP manual for more date options: http://php.net/manual/en/function.date.php
Hope that helps!