I'm not sure the description's expectation for the Limits ... apply to service renewals options are accurate to what we'd want.

It seems to me that the problem here is that we want the ability to edit a service to save the service data without causing an error about the coupon being invalid due to limitations. In this case, we can solve that problem by only applying the coupon validation iff the coupon has been changed.

We would need to have the same occur during upgrades/downgrades. Unless the coupon is changed during the upgrade/downgrade, there is no reason to validate it.

The primary problem here is in answering the question, "When does the service renew?" so that we can then, and only then, validate the coupon again based on its defined settings.

As far as I can tell renewal happens exclusively through the cron. It seems to make sense for the coupon usage to be incremented at the time of invoice creation.

So this is the way I would expect coupon usage to act:

Service creation will increment the coupon usage for either 'limits ... apply' setting.
Service renewal will only increment the coupon usage and validate the usage limitation for the 'limits do apply' setting.
Service editing will only increment the coupon usage and validate the usage limitation when a coupon is being added or changed. (IMO this is what should be done to complete this task, just not tying to increment at all rather than not validating)

Should coupon usage be decremented when a coupon is removed from a service?

Another thing not directly addressed by this ask but related that since renewal invoices are created by the cron, they will fail silently once the coupon reaches it's limit. Even just ignoring the coupon sounds better than that to me.