I'm 100% certain Aziraphale and Crowley will have their happily ever after. Here's why, everything has already happened. "It starts, as it will end, with a garden. In this case, the Garden of Eden, and with an apple," God, season 1 episode 1. God is telling Crowley and Aziraphale's love story because she's their biggest fan. She's been there for every moment of their relationship, from their starry angelic meetcute to all their reunions which were mostly accidental at first, then increasingly planned and desired. She knows everything they do down to the smallest detail, like all their secret rendezvous, The Arrangement, that they've been meeting in St. James's park for a very long time. She probably analyzes their arguments like anyone else committed to their OTP and celebrates every time they remember how important they are to each other.
I think that if the show sticks to what was promised--the ending in a garden, then Aziraphale and Crowley will pledge their love in the garden in front of witnesses like Jesus, whose second coming is less bloody than expected and who is very happy to see the demon who showed him all the wonders of the world. Maybe he'll even help Crowley rescue Aziraphale.
I also think that God will speak to Crowley and Aziraphale, using Crowley's angel name, which will change him back into an angel. Names are very important in this world.