Hmmm. Well, I believe it depends on the type of WMD attack, if it is successful, etc.
I can give you some possible knowns, which may help answer the first question.
1) A response by Israel would have the potential to cause Allied casulties, depending on the response (depending on location of Allied forces, etc.)
2) Arab countries such as Jordan, Syria, Saudi Arabia, Kuwait, Egypt, Turkey, etc. want Saddam out of power, as they believe he adds instability in the region and threatens their own sundry interests in the region. And they also want Saddam out so that the US leaves the region.
3) A military response by Israel on Iraq which happens to kill civilians could cause an uprising in the populations of the countries mentioned in #2, not to mention fomenting additional anger in several terrorist organizations. However, it may be such that *any* military response by Israel to a WMD attack by Iraq would not be tolerated by the aforementioned countries.
I believe that Israel would respond immediately if nuclear weapons were used in an attack by Iraq. Biological and chemical - I do not know. They may wait to assess how Allied forces are doing. What I would not want is a response by Israel that harms Allied Forces.
As to the second question - leaders of countries in the region will need to ask themselves if they want to hitch their wagon to Saddam and go to war against Israel (if they respond to an attack by Iraq). A war between them and Israel would set these nations back for decades. Is it worth it for them to do so? I do not think so, but I am not an Arab and/or practice Islam, which I think are two main driving forces in the region. But I am a capitalist and although these countries may despise the US, they crave US dollars.
A couple of additional points. Any attack using WMD by Iraq will set France back in the international community for years. They may say "This is what we were trying to avoid!", but reasonable people should conclude that Iraq did not want WMD found, and France knowing that, was complicit in further delays. I think it reasonable to posit that delay caused by France's gyrations have made the region even more unstable.
I still consider Russia a widlcard in all this. Russia certainly wants US dollars as well and they also want the billions of dollars of business that they have ongoing with Iraq. Russia has had their own tremendous problems with Islamlic fanctics and I believe the US will take special care of Russia to ensure they are compensated for lost revenues.
I do not believe that NK is any more apt to attack SK once an attack starts on Iraq. They would presume that the US would be *less* prepared to defend against a NK attack once an attack starts - and there are more forces in the Korean area, as compared to one week ago.
I rambled and my apologies.