Yes, Premast Plus will be on Google Plus soon enough, as a team we are working on the Google slide version right now.
Keynote does not have the ability to use software plugins...But we are working now on another solution for Mac OS which can be used on Keynote somehow
Next step, we are working on this as a priority and it will be released this year.
No there isn't, you can use it as many times as you like and download item as much as you can once you install it.
Yes sure illustrations on Premast Plus are fully editable, you can select the item you want and click on "convert it into shapes" then ungroup it and you can edit it. or you can check this detailed tutorial
You totally can, just contact us on firstname.lastname@example.org and we will inform you with the exact steps to become a one