Will Israel survive the present alliance between Russia and Iran?

Maybe. This is the only useful answer here.

Will the U.S. go to war with Iran if it knows Russia will back it up with nukes?

No!

Will the U.S. go to war with Iran if Russia threatens nuclear weapons and Iran threatens or destroys Israel with Nukes?

Unknown but unlikely.

The U.S. has a vested interest to survive as a country.

Israel has a vested interest to survive as a country.

But, in the present dynamics of the world the U.S. likely would sacrifice Israel not to blow up the whole world and to kill all life upon it permanently in a nuclear war.

This is a given.

So, will Israel survive the next 20 to 30 years?

So, the nuclear peace treaty with Iran has more to do with it's alliance with Russia than any other single factor.

Israel's problem is not the U.S.  It is Russia and Iran together. And this is also the problem of the entire Sunni world in the middle East. The problem is Russia and Iran.

ISIS(ISIL) is completely a reaction to what Russia and Iran have done in Syria and Iraq.

If the world understands this better a nuclear conflict might be avoided some time in the future between various nations here on earth.

They might need an alliance with Russia in addition to the U.S. to accomplish this feat.

But, the possibility of that might not be very great.

Right now, Israel is allied with Saudi Arabia and the U.S. and the European Union. We will see what happens next.