I think this is a really tough question. For me, I think it's a mixture of both #4 and #5. I say this because the ancestry and the horcruxes (and the reason that Riddle/Voldemort wants them) all tie in closely together.
The two ways that I would imagine they could be worked out by Mr. Yates both won't really work — Dumbledore can't explain it and Voldemort could explain it in his evil way, but we won't see him till the end — so, the only other way that I think would be kind of cool is if they use Dumbledore's portrait. But, even then, Harry won't get to Hogwarts until the end. J.K. Rowling has always used Hermione to explain things, but Hermione has no information on much of this (i.e., how to track down the horcruxes). So, I guess, in the end, there are two ways that can perhaps work out:
1. Mr. Weasley explains things to Harry
2. Something from RAB (Regulus Arcturus Black) that gives more information once Harry figures out who RAB is
Both of these things could happen towards the beginning of Part I. I know that many people were confused about the horcruxes in HBP. In fact, at the press conference in NYC, I had to explain to a few reporters what the big deal was with them. So, these things really need to be explained at the beginning to help those who were confused and those who haven't read the books.
The burning of the burrows was indeed a surprise and, I think, a waste of time, money, and scenes while other important scenes were omitted in its place. To fix it, well, being that they ARE witches and wizards, I imagine they will use magic to put the burrows back in order.
Meanwhile I would imagine Yates adding Fleur and Bill's wedding in the story by using the burning of the burrows as an excuse for Bill to be there and then add in an "oh, by the way," Fleur and Bill have been dating since the Triwizard tournament. Being that Deathly Hallows will be a two-part movie, it has the advantage of time but not much to add that in. They still need to add a bit more about the relationship between Lupin and Tonks, hence the end of the book (DH).
The big fight scene could definitely have been more enhanced in HBP, but I don't think it is that necessary for HBP because there will be so much of it in DH. Since they lightly tread on the horcruxes, as crucial as it is to DH, I imagine he will find a way to have Harry figure out what, and possibly where, the horcruxes may be by looking further in to Dumbledore's pensieve collection.
I applaud David Yates for taking on the responsibility to convey the final chapter of Harry Potter. It is indeed a challenge to exemplify so much detail in such a short time frame, even if it is broken into two parts. I feel confident David Yates will be able to pull it all together and not disappoint.