1) What if Wharf didn't have that +buy, but Merchant Ship did?
Donald says Wharf got the buy to compare more favorably with MS... was it truly necessary? Certainly, Wharf's buy is very useful, but I doubt Wharf would really fall out of the top 10 $5's without it, or not by much if it did. And Merchant Ship would certainly benefit - that next turn essentially now has a free Grand Market thrown in. But I still think Wharf would be a lot better than MS.
I like these cards as they are, but at the same time I think that Donald's reasoning for which one got the +Buy is one of the very few mistakes he's made. To clarify: The final cards are NOT mistakes, but I think Donald's stated reasoning for how they should be is incorrect. Because without any +Buys, Wharf would still be the stronger card.
Why? The short answer is because Wharf's next-turn effect is essentially a non-terminal draw rather than a terminal draw, which makes it much more powerful.
The more detailed analysis: In general +$ and terminal +Cards are roughly equivalent in power. Which one is better in any given instance is situational, but figure that they're the same when $1 is your average card value, and that's often what it is. If you look at the other official cards, though, you can see how Donald has designed it so that +$ is treated as slightly stronger. Compare Moat to Duchess, for example. Moat offers +2 Cards and a bonus. Duchess offers +$2 and has other effects that cancel each other out entirely. Notice that Smithy only costs $4, but a terminal Gold, if it were to exist, would probably cost $5. Torturer, Rabble, and Margrave offer +3 Cards and attacks, but +$3 cards at the same price level are finicky (Mandarin) or unreliable (Harvest).
It makes sense, then, that a double +$2 would be stronger -- or at least costed as if it were -- than a double +2 Cards.
However, the fact that one of the +2 Cards bonuses on Wharf is non-terminal is HUGE. It's Laboratory vs. a non-defending Moat. That's simply staggering. By contrast, while vanilla +$ bonuses are certainly better if you don't use up an action to get them, they don't increase in power
as much.
Thus, if neither card had any +Buys, Wharf would still be stronger than Merchant Ship. If the goal, as it seems to have been based on what Donald said, was to balance the cards out in terms of power level, the +Buys really should have gone on the Merchant Ship.
But like I said at the outset, although this was probably a mistake of the reasoning, I don't think the end result was a mistake. It's not all that important for every $5 card to be equivalent in power level. It's nice having Wharf be powerful, and Merchant Ship isn't so weak from lack of +Buys that it isn't still a worthy member of its cost category.
Additionally, there are more cards that offer money with +Buys than cards with +Buys. So the published versions of Wharf and Merchant Ship cards diversify the card pool a bit. To put it another way, if the +Buy were moved to Merchant Ship, then Merchant Ship would be a double Woodcutter, and Wharf would be a double Moat. As they are now, neither card really has an official "this turn only" equivalent, and that's more interesting.
2) What if Ambassador cost $4?
Opening double Ambassador is often a very strong move, and Ambassador itself is very powerful. The jump from $3 to $4 is not huge, so preventing the double Ambassador opening and not much else doesn't seem like it would ruin the card or the card's availability much. Ambassador would still be a top $4, methinks, but would probably open Ambassador games up a little.
Here, however, I think it was a small stroke of genius to cost it at $3. As you say, Ambassador would not play THAT differently at $4. And since it's such a strong $3, you'd think it "should" be $4. But in terms of practical effect, the only things I see a price hike doing is closing the strategy space. At present, Ambassador/Ambassador and Ambassador/Silver are both really strong openings, and we've had much debate and analysis on this site for trying to figure out which one is better in which situation. But costed at $4, Ambassador/Silver would be a pretty automatic opening; in other words, a lot less room for strategizing.
MasterAir makes another point I hadn't thought of, which is that a $4 Ambassador would also be just slightly more luck-based in terms of picking up a second copy on the second shuffle.
On the other hand, a $5 Ambassador shows more promise. Not saying it's necessarily an improvement on a $3 Ambassador, but it does not suffer from the strategy-closing problem of the $4 Ambassador, because now you can't open EITHER Ambassador/Ambassador or Ambassador/Silver, so there is no "automatic" opening. It might still suffer from a luck problem, in that it might make a 5/2 opening dominate a 4/3. But that wouldn't be anything Witch and Mountebank don't already do.
3) What if Adventurer cost $5?
I've wondered this myself. It's not broken at $6, but I don't know why it would be broken at $5. Like you, I'm not really saying it SHOULD be changed, but try as I might I can't shake the hypothesis that maybe, just maybe, a $5 Adventurer would be a good idea.