Your first question mirrors the age-old one of which came first—the chicken or the egg? Just as you seemingly cannot have one without the other, you cannot prove one without the other. It is a classic “Catch-22” situation. Similarly, as there is nothing definitive to prove or to disprove that God exists (i.e., there is no available evidence, nothing, either way…), then anyone who claims that he or she can do either is “going beyond” nothing, which equals the available evidence, and which is still nothing (0 + 0 = 0). Alternatively, though, if we choose, we can agree with the astrophysicist, Neil deGrasse Tyson, who answered, “The egg—laid by a bird that was not a chicken."
As to your second question, the definition of a subject complement is a noun, pronoun, or adjective that follows a linking verb. While a linking verb is present (is, a form of the word, be), the words “going beyond” are not a noun, a pronoun, nor an adjective. By definition then, those words cannot be a subject complement.
The verbal phrase, “go beyond,” which means to do more than (something), or to do more than is expected or required, has different forms (tenses) to denote the passage of time (went beyond, gone beyond, goes beyond, etc.).
Your sentence, which I will truncate (“Anyone who claims there is no God is going beyond the available evidence.”), does not use auxiliary verbs, and does not refer to a specific time, so it cannot be in the Simple tense. Similarly, it does not use the words have, has, or had as auxiliary verbs, and it does not allow action to continue over time, so it cannot be in the Perfect tense. However, it does use the words—is, are, was, or were as an auxiliary verb, along with an –ing ending on the main verb, and it does focus on the “progress” of an action, so again, through the process of elimination, we can say it is in the Progressive Tense.
Part I of II