As to the attack by the US... well, it cannot be prevented if the US is committed to carrying it out, but if those Senators confirm the rumours the EU diplomats should remind the US that is an illegal action.
It could possibly be prevented, but it depends on how badly European nations wanted to prevent it. More on this in a bit.
You ask If an attack on Iran is allowed to happen, what are the chances that Europe will escape being drawn into a greater conflict? but I don't see why the EU would be attacked by Iran or its allies...
You answer this yourself in regards to Iraq. You write, "A spillover of violence from Iraq would be of concern to Europe, the Middle East is relatively close and accessible". Spillover from Iran. Even if it is attacks on Americans in Europe, it will be messy.
So could Europe block an attack on Iran if Europe or Russia aligned itself with Iran? I do not know. Would it make the situation too hot, or would the U.S. think twice before risking war with Russia or any number of European nations and go ahead and attack Iran?
If the U.S. is not prevented from attacking and invading Iran, what is to stop the U.S. from continuing these invasions? Internal U.S. collapse?
Is U.S. being appeased by allowing it Iraq? Will it stop with just Iraq?
I have been convinced that yes and no for years.
The US hasn't put itself on a war footing internally (economically and politically) as it did in WWII. I think the US has the potential to go on for much longer. Especially if the public can be told that the rest of the world is out to get them, which is what they would be told if NATO allies allied with Russia to block an attack on Iran (or after such an attack). Eurabia, the Russians and the Chinese are coming! Oceania has always been at war with Eurasia! Like I said above, where the UK's chip would fall is an interesting question. When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes
There isn't the industrial base needed for a war footing. Not that the next war would necessarily be industrial - at least, not for long - but politically the population wouldn't support a war without physical coercion, and physically the US doesn't have the energy needed to re-industrialise itself.
Boeing, Northrup, Lockheed, Ford and General Motors might beg to differ. The one thing we do still have is the war making ability. It might take a coup to bring it off properly, though. Especially if the Dems win in the fall. If the Repubs win that might itself constitute a de facto coup.
Victory would probably only be possible through massive genocide and the environmental consequences would likely make it pyrrhic--nuclear winter if things went nuclear; world wide plagues if biological; massive genetic damage if chemical. If the US went for control of Iran it is hard seeing Russia and China taking this quietly. Plus we would have to deal with Pakistan and Afganistan. The religious would truly have reason to hope for the rapture.
What would prevent this is simply lack of sufficient military capability. To bring this off it would be necessary to double the size of the ground forces. That would take more than a year. With the Bush administration expiring in seven months about all we could do now is inflict the maximum damage from the air. They might do that, figuring that that would get McCain elected and commit him to finishing what they had started. Stay tuned.
That we are having such a discussion is itself an eloquent explanation of why I refer to us as a nation of dumb fucks. Dumb fucks with a taste for psychopaths.
As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."
[AR Geezer's Crystal Ball of Doom™ Technology] When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes
