From your formulation of pinciples, 1) with "multiple cards resolve" sounds a bit off, I think "multipele effects resolve" would be the better formulation.
Let's start with Q1.
The correct answer is A2 (current player gets the one remaining Curse). Turn order is turn order, even if the effect originated from a card not belonging to the current player.
Let's move on to Q2.
All your detailed event listings are wrong, so the correct answer is something else. A2 seems to the nearerst to the truth.
After A returns 0-2 Embassies to the pile, the other players are supposed to gain one each. Since players can't gain cards from the same pile at the same time, this is broken up in player order (just a for Q1). So B gains an Embassy. Normally the game would proceed to C gaining an Embassy, but something triggered. So the execution of Ambassador's effect is suspeneded until everything that triggered from the gaining is handled. And that includes things that trigger from handling those triggers,a.s.o.
Since B gained an Embassy, all other players in turn order gain a Silver, so A, then C then D gain a Silver. Nothing more triggered on the Embassy gain by B and nothing triggered on the Silver gains, so the interruption of Ambassador's effect is over and it continues with C gaining an Embassy, triggerring again a round of Silver gain.
So the full order of effects on that playing of Ambassador showing an Embassy is
A4: A reveals an Embassy, A returns 0-2 Embassies to the pile,
B gains an Embassy, A gains a Silver, C gains a Silver, D gains a Silver,
C gains an Embassy, A gains a Silver, B gains a Silver, D gains a Silver,
D gains an Embassy, A gains a Silver, B gains a Silver, C gains a Silver.
This oderering is completely forced, no choice involved. It also assumes that there are enough Embassies and Silver available to fulfill the demand.
Now let's add some fun and assume Changeling is also in the game. Then for each Embassy/Silver gain, there is an additional optional trigger: Exchanging it for a Changeling. The interaction with the Silver gains is easy: Changeling is the only thing triggering of the Silver gain, so it is has to be handled immediately after the gain.
So in the above order of events the first Silver is gained by A (after B got the first Embassy). Now, before that game proceeds to give C the next Silver, all triggers on the Silver gain by A habe to be resolved, so A has to decide if they want to exchange the just gained Silver for a Changeling. Only once that is decided and executed (if so chosen), does the game proceed to C gaining a Silver, and then C has the same decision to make, a.s.o.
How Changeling interacts with the Embassy gains is more complicated. Once for example player B has gained an Embassy, now 2 things have triggered:
1) the mandatory Silver gain for the other players by Embassy itself, and
2) the optional exchange for a Changeling.
Both triggers 'belong' to player B, so they get to decide which order they are executed in. They can choose to do 2) before 1) and thus exchange the Embassy for a Changeling before player A (and later C and D) gain their Silvers. That may be prefferable if there are less than 4 Changelings left and B fears that A,C and D might all exchange their Silvers for Changeling.
But it's also possible to do 1) first and 2) later, which has the benefit that B can make a choice based on what A,C and D did.
To sum this up, each triggerable effect has its own queue, where effects happen by affected player in turn order, and each affected player decides upon the order of their effects. If new things trigger, they interrupt the execution of previous effects until they are dealt with, then the previous effects continue.
An example of the last sentence, the possible exchange of the Silver gained for a Changeling interrupted the Silver gains from the Embassy trigger, which in turn had interrupted the Ambassador giving out Embassies.
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Let's move on to Eavesdropper.
It's clear (I think) that 'waiting' on a gain and not calling your Eavesdropper isn't helping here.
So A buys, then gains a Grand Market (GM0), 5 Grand Markets left (I'll call them GM1-GM5 from top to bottom).
Eavesdroppers for B and C trigger; B is fírst in turn order, so they call an Eavesdropper and gain GM1
For the GM1 gain, Eavesdroppers from A and C trigger, and they interrupt the triggered effects from the GM0 gaining.
A is first in turn order and calls an Eavesdropper to gain GM2
For the GM2 gain, the Eavesdroppers from B and C trigger and they interrupt the triggered effects from the GM1 gaining.
B is first in turn order, they call their second Eavesdropper and gain GM3
For the GM3 gain, Eavesdroppers from A and C trigger, and they interrupt the triggered effects from the GM2 gaining.
A is first in turn order and calls their second Eavesdropper to gain GM4
For the GM4 gain, Eavesdroppers from B and C trigger, and they interrupt the triggered effects from the GM3 gaining.
B is first in turn order and calls their third Eavesdropper to gain GM5
Now the various layers continue, allowing players to call their Eavesdroppers as indicated above, but that is pointless, as there are no more Grand Markets to gain.
Once the dust settles, A has gained 2 and B has gained 3 of the contested 5 Grand Markets. C, being last in turn order, is unhappy now.
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Your additional questions:
1) Note that the first printed version of Moat also doesn't say that it happens first:
http://wiki.dominionstrategy.com/index.php/MoatJust like for Secret Chamber, it was assumed/understood that those attack-centered reaction effects naturally had to be done first, because especially for Moat it doesn't make any sense otherwise. When the game evolved and had Reactions that were not related to attacks any more, it became clear that the rules didn't really support the way Moat etc. were supposed to work. As a result, the 2E printings of Moat, Beggar and probably others were changed to include "first" in their wordings. Since Secret Chamber isn't in 2E Intrigue, it didn't get the wording update. If you play with it, I'd suggest you react with it as it was always intendend: It happens before the attack effect.
2) That I'm slightly unsure about, but I think the central rule of doing things in turn order still applies.
So once you have 'passed' on doing something "first" when A played an attack, your chance is over to do anything for that attack. As Donald X.'s reply below makes clear, player B can use Diplomat again.
