Customizing Jetpack comments

Since Jetpack’s Comment form is added to your site via an iFrame, you won’t be able to customize the form itself (its look or contents), that lives on WordPress.com and appears inside the iFrame on your site.

You can, however, change where the iFrame is added on your site (that would be your second question). You can do so by editing your theme’s comments.php file, and changing where the comment_form function is loaded. Here is an example with Twenty Seventeen:
https://github.com/WordPress/twentyseventeen/blob/master/comments.php#L79

As you can see the function is added at the bottom of the file, after the function that renders the comments list (wp_list_comments). That is most likely how your own theme is built. If you want the comment form to be displayed before the comment list, you will need to move the comment_form function in the file.

I hope this helps.