In what way is the Resurrection the best explanation?

[GrandizerII]

According to Bart Ehrman, the suffering servant and resurrection ideas were artifacts of what the disciples observed while also wanting to continue to believe Jesus was the Messiah.

Agreed. No-one expected the Messiah to suffer. However after the Resurrection, they went back to the OT sources like the prophets, with passages such as the suffering servant in Isaiah, and realised the clues had been there all along. I tend to use this as evidence for the resurrection, so I'm a little puzzled here.

Non-Christians Jews would disagree, and many interpret the suffering servant passage to be referring to a nation, not a particular person.

And the Resurrection didn't have to actually happen for the disciples to reinterpret the OT passages, only the belief that it did. So no what you're saying is not evidence for the Resurrection more than it is evidence that the belief in the Resurrection arose from C1 Jews, which no one actually doubts.

Perhaps you should state here what the promise was exactly. You're referring to the Genesis promises to Abraham, aren't you? If so, you'll see that the promise hasn't exactly been delivered if we take a good look at the wording within context.

Things like the Blessing to Humanity, father of many nations, fulfilling the Covenant, sorting out that nasty business in Adam...

Except these are pretty much vague terms that don't make for good prophecies, and nonbelievers have no reason to believe these have occurred. When you want to present evidence for your side, make sure to present evidence that we can all observe (or refer to independent "almost unbiased/neutral" primary sources, at least) because such associated beliefs are not evidence in this case. These are Christian beliefs that we do not hold.

I've reread the relevant passages and I'm not sure problems what you're referring to. Remember that Paul and the whole of the (Jewish) Earliest Church were able to claim the promises had been completely fulfilled, so I would think more modern believers should simply follow the same route.

The problems are clear to me as the sun (mainly that they are vague and very ambiguous), but I understand that you yourself wouldn't see these problems.

This is also a bit of a mischaracterizing of what I actually suggested in the OP.

Apologies if I've misunderstood. I think therefore it would be helpful for you to flesh out what you are saying. Could you provide a route map from Good Friday to Paul?

I don't think there's any need for such. The OP provides an alternative account which covers the initial bases that would otherwise lead to the Resurrection as the conclusion (even if, like the Resurrection, is not really something we know happened). But again, I have to repeat: at the end of the day, we're still going with very biased sources here that contain all sorts of claims not exactly documented elsewhere, especially the Resurrection. The OP was an attempt to nevertheless arrive at a different conclusion while virtually not rejecting the early Mark account of the Resurrection and treating the later Gospel accounts of the Resurrection as reinterpretations of early Mark's account. Whether it was a mistake to go about it in such particular way, the more important point still stands: the Resurrection hasn't conclusively been established as the best explanation.

In particular:

For C1 Jews, the hot topic was the arrival of the Kingdom of God. The internet forums of the day were stuffed with threads about when it was coming, what it would look like, who would be in it etc.

Then there was this rather vague figure of the Messiah (Anointed One) who would bring in the KoG. Anyone claiming to be that was destined to succeed, because God was with them making sure it happened. Therefore, those who failed could not by definition be the Messiah. Their death said, powerfully and irresistibly, this wasn't the Messiah and the KoG hadn't arrived.

That's the theory- it worked like that in practice too. Josephus' writing, Gamaliel's speech in Acts 5, the first and second Jewish rebellions- failure is not an option. Jesus ended up dying exactly like the other failures.

So if the Earliest Church were saying Jesus was the Messiah, he must have succeeded. How?

Fast forward to Paul, writing 20/25 years later. The Resurrection is so well agreed within the Early Church that there is no trace of debate in his letters. Indeed, he uses it as a fixed point from which to deal with the highly disputed issue of Torah obedience.

Paul claims not only that Jesus is the Messiah, but that the Kingdom of God has been inaugurated...Abraham...forgiveness... etc. The reason we know this has happened is because of the Resurrection, he says. And it's clear from his writing that this has been a thing from long before he wrote. All seems to fit together.

And all fitting together with secular analyses as well. Like others have said, and like I said earlier in the post, the Resurrection didn't need to actually happen, only the belief that it did. Bart Ehrman in fact has written a book explaining how it all happened, and I still advise you to read that article I linked to earlier.