1. This is an excellent point you make. There is, a priori, very little reason (as far as I know) to assume that an alternative universe would have the same physical properties as ours. If there is evidence in that direction I doubt it's merely "because our universe has this property."
But I agree with you in principle that this is a weak point. The essay's point was to respond to pg's attacks. jsomers never assumed that pg's single paper had somehow thrown philosophy into chaos and shambles.
2.I agree with you here that there is little distinction between these fields in Greece. But that's an interesting fact isn't it--it's essentially conceding that at least initially philosophy played an important role in the development of math and science (one certainly can't assume that the development of philosophy and math were going in two different tracts in the same man's head).
3.He's not saying that philosophy failed. He's arguing that pg is finding fault ex post facto. It's easy to fault Kant after you've read a response but perhaps without Kant the response (and the progress that supposedly came with it) never would have occurred.
For an analogous situation--Galois developed his theory which allowed for a proof that no polynomial of order 5 or greater was guaranteed a solution by radicals. Does this mean that Ruffini was a putz because he could only do most of the proof for order 5 polynomials? After all--he was clearly taking the wrong approach.
Or, do we think Pascal is an idiot because Newton developed the calculus that solved the problems he could not? No.
The point is that everything builds on everything. I think somers' Kant tree is a great example of this.
4. This is backwards. The point is that philosophical reasoning often led to the creation of entirely new fields of thought. Economics, politics, law and linguistics are great examples of this.
Would we have modern legal thought without the works of philosophers like Locke, Berkeley, Hume and Hobbes?
Would we have modern economics without the work of Smith, Pareto and Mill?
Would we have modern sociology without Marx and Gramsci?
That they called themselves "philosophers" is not irrelevant whatsoever. It demonstrates that philosophy often lays the groundwork for entirely new fields.
It's strange you say that their results did not require the existence of the rest of the field of philosophy. I suspect they would disagree as they seemed to rely on other philosophers in their own work.
5. What is your metric for "useful"?