I am not fully aware of the possibilities of cf7 or your abillities to change it. But when I come across a situation like this i find it more user friendly that your page scrolls back down.
You can do this by adding an anchor to your post of the form like #contactform , so when the page refreshes to …#form it scrolls back down. I think this will be easier to achive than to go change to plugin to have it in some modal popup.
Also when users have to change something in the form, they are already back at the form.
check out http://www.w3schools.com/tags/tag_a.asp, or just google html anchors for more info. and add the needed changes to the plugin form.
http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_link_bookmark