Re diplomacy, what's to stop them simply capturing the lot of us and offering us as sacrifices to the wyvern? There's a lot more of them than us, and it's not unlikely that they'll prefer not to look a gift sacrifice in the mouth rather than take a chance of us actually killing the wyvern. Also, they're goblins - even if the scenario plays out as hoped, there's a decent chance they'll turn on us as soon as the wyvern is dead. I must admit, I remain sceptical of our chances of success using diplomacy.

Overall, I feel there's two factors that complicate pretty much any plan that involves directly interacting with the goblins, including my own suggestion:
1. We're heavily outnumbered - they have a very strong chance of cutting off any escape route we plan on using.
2. If we manage to get away - with or without hostages - they can just come after us. They may not like sunlight, but they're can actually operate in daylight just fine.

So, basically any plan we come up with has to solve these two issues to have a decent chance of success. If distraction in some form is part of it, the distraction must be big enough to keep them from using their number against us and keep them from getting their act together and running us down.

I do agree that waiting to move at night is not a good idea - that's will hand the goblins an even greater advantage over us. Even just doing a stake-out is basically the same as waiting for them all to wake up and become more active.