As a SendGrid Support Technician, I can tell you our Parse API retries for 72 hours, same as our outbound mail management. We use POST params, not JSON: http://docs.sendgrid.com/documentation/api/parse-api-2/
Our Parse API is complimentary, and doesn't use any credits.
We also have POST pushing of live data such requests received, deliveries, bounces, clicks, and opens down to address/link/time-specific detail. This is offered as either JSON strings or POST params: http://docs.sendgrid.com/documentation/api/event-api/
My only problem with your model is that it requires me to be already paying you a non-trivial amount of money (for a brand new app) to get started with your Inbound API.
We also have POST pushing of live data such requests received, deliveries, bounces, clicks, and opens down to address/link/time-specific detail. This is offered as either JSON strings or POST params: http://docs.sendgrid.com/documentation/api/event-api/
- Jacob