If I want to use MongoDB in my ecommerce app no worries. If I want to simply use MongoDB offer some minimal improvement or added functionality, call it ZongoDB and sell subscriptions... not so fine?
No, because such a license would - by definition - not be an Open Source license. That's basically what the Common Clause says, but a license with that clause attached isn't OSS.
No. Open Source always implies freedom to use for any purpose. There might strings attached such as the need to open-source modifications as well, but if usage itself isn't free, it's not Open Source.
If your web app uses MongoDB (or any SSPL licensed software) to provide a value that is not substantially MongoDB itself, you are not making the software available as a service. If your code facilitates making MongoDB's features available to other users, you are providing MongoDB as a service and are obligated to make all that code available under the SSPL.