You seem to have sent your reply already, but I'd still like to add to the framing aspect as follows (I also think your reply is too long, but that's another issue):

As you and MillMan already pointed out, Dunphy skips facts and argues emotionally, by asking readers to view your letter through the America=Good prism.

This will always work, simply because people in Western Europe do have a lingering "America is better than Russia" attitude, and putting a few facts down cannot change that. In many people's minds, 1. America, 2. Russia.

I'm unconvinced that you win the argument by trying to ignore this point, because it speaks to people's fundamental self-interest, whereas you're ("merely") speaking to people's sense of fairness in argument.

In the end, America has been and still is the formal guarantor of European security. No amount of direct arguing is going to bring people to the alternative idea 1. America, 1.Russia, ie that America and Russia should be measured with equal weights, or god forbid 1.Russia 2.America.

However, you can (IMHO) win by appealing to people's self-interest. The only thing that trumps America's 1. position is Ireland's (or possibly Europe's if you like) 0. position:

0. Ireland.
1. America.
2. Russia.

To apply this approach, you'd have to counter accusations of anti-Americanism not by denying them, but by showing their logical equivalence to pro-Irelandism. Most readers of Irish newspapers will not put America before Ireland, they can only do so if America's actions are seen as good for Ireland, or if Ireland is not mentioned in the debate. By keeping the debate with Dunphy on the America/Russia plane, you're not using this.

Dunphy's underlying argument is "don't attack America, because America is good for us". If you reply "X is good for Ireland, America is not doing X", you can be (more, IMHO) persuasive. Moreover, if it so happens that X implies a more even handed America/Russia point of view, that can't be helped, IYKWIM.

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