If you would otherwise hit your next shuffle before end of game, it helps by putting off that shuffle. Even shuffling average cards in can help if you're leaving behind enough chaff. And as for #5, it's fine to have more than 5 cards you don't want to shuffle in. Let's say I have 6 Victory cards in my discard pile. I'll shuffle in one of them if it means also shuffling in a bunch of good stuff. Plus the Duchy you gain misses that shuffle as well.
It doesn't matter whether or not you're delaying that shuffle if you're shuffling in average cards. If you let that shuffle happen naturally, you still shuffle in average cards so it makes no difference. You need to be shuffling in stuff that's better than the average card on average, and if you're taking two debt for it, that difference needs to generate more than
and it'd better generate it fast because you have to take the debt immediately.
As for #5, it's specifically referring to cards that you don't want to be shuffling in. If you're fine with shuffling in one card alongside a bunch of good cards, then you're fine with it. The point is that it's not good enough that you have a bunch of crappy cards and a bunch of good cards so you get to select just the good ones; you only get to filter out up to 5 crappy cards so all the remaining cards combined need to make it worth it.
3D to make 5 cards miss the shuffle, that's better than Expedition.
That's not how costs work. Annex doesn't cost
for the Duchy and
for the effect, it costs
for the whole thing, which is worse because it forces you to get both. Normally, you would very rarely buy Duchy and Expedition for
and two buys, because that's enough for a Province. Instead, you can buy Expedition when you can't get or don't want Duchy in order to reach for Province, which is something that you can't do with Annex — on the contrary, Annex makes it more
difficult to reach for the Province because it gives you debt that you have to deal with on your following turn.