Concerning angels at the time of creation, I am aware of the fact that that concept entered into rabbinic vogue over time, but as far as I know there's no evidence that that was the dominant view (or even a view at all) during the eighth century B.C.

Concerning a triune Godhead not being able to be alone, I'm not sure I agree. Just for the sake of reference, the orthodox trinitarian doctrinal formulation is that God exists eternally as one substance in three persons. Thus, while any of the three persons certainly cannot be said to be alone, the ontological substance of the Godhead can be. An illustration may serve: let us suppose that I belong to the only church in the country, and that this church has one hundred members. When we are ***embled for worship, not a single one of us can be said to be alone, for we are surrounded by brothers and sisters in Christ. And yet, as a church, we are alone, for there is no other church in the country. In like manner, while there is fellowship among the three persons of the Godhead, nevertheless insofar as He is God, He is alone, for there is no other God (or any other being, unless you contend that the angels had already been created, for which there is no biblical evidence) with whom to commune. Of course, the ****ogy breaks down since "members" of a Church and "members" of the Trinity aren't entirely parallel, but for the purposes of this discussion, I think it functions well enough.

Finally, concerning the use of the term "alone" (Hebrew, "bad"), my contention is that this p***age does not use it to refer to loneliness or to the state of being by ones' self, but instead uses it as a qualifier of the verb "to create." It is not describing the condition of God, but the condition in which creation is being carried out: by God, and only by God. This is true whether or not God is in fact alone, and whether or not He is able to be alone. Another illustration: imagine I am taking a course along with twenty other students, and that I have done poorly on a test and must thus retake it. I might accurately say "I alone had to retake the test." Though surrounded by fellow cl***mates, and therefore not in fact alone, I am still using the term correctly since no one else engaged in the act of retaking the test with me. Now, let's change the scenario slightly. Imagine that cl*** has been canceled but no one told me, and thus I am the only one to show up on Monday morning. Again, I might say "I alone showed up for cl*** today." In this case, I am in fact alone, and I alone have shown up for cl***. One more scenario shift: let's say that myself and two other cl***mates mistakenly showed up for the canceled cl***. Though none of us is alone (since there are three of us), it is nevertheless still legitimate to say, "we alone showed up for cl*** today." I contend that this is how Isaiah uses the term bad in this p***age. Whether or not God Himself is indeed alone is irrelevant to Isaiah's purpose; he is not commenting on that, he is instead pointing out that the creation of the world can only be attributed to God since no one else participated in that act with him.