Is the 100mb upload limit on free accounts per month, or total?

Additionally, if I pay $19/month for 10GB, is that my total max capacity? Am I able to clear out space for continued use. Once I hit 10GB is my jot form no longer usable?

The available upload space is in total, this is the only limit that does not reset monthly, if you reach the limit you will be notified, your forms will be disabled and you will need to upgrade to get more free space and continue using your forms. 

Also, to clear this space you will need to delete those submissions where you did receive uploads, this is why I would suggest you to make a backup so you will avoid to loose some data, there is a new feature that will allow you to download all your submissions and data as well as the forms as HTML files, here is the link where you will find the steps to download your data: How-to-export-all-of-your-data-at-once

So once you do that, you will not be afraid about loosing some data, because you already downloaded all your data and you can delete those submissions and clear the upload space in your account. 

Hope this helps.