Thanks for engaging in my post! You have struck upon an issue that has kept the philosophers of biology in debate: how should we define the terms adaptation, fitness and so on in a way that avoids tautology. Generally scientists do not really engage with semantic debates, but this one is a bit of an exception because of the rising challenge of Creationist objections to Darwinism and the occasional confusion on a practising evolutionary biologist.
The common resolution, as discussed by Gould (1976), is that adaptation does not necessarily equate survival but rather means ‘design’ (which he attributes to Darwin’s original argument). Using an engineer’s criterion of adaptation-as-good-design is one resolution. The implicit problem here is that realised ‘good designs’ are only a fraction of possible ‘good designs’ that an engineer could imagine. As Gardner (2009) argues, we know the reason for this is because we now know the ‘design criterion’ that natural selection selects organisms to maximise — namely, inclusive fitness (see also, West & Gardner 2013). In this way Gardner, in these articles and elsewhere, develops strong argument that adaptation should be equated with the appearance of design.
And, I would add, that survival is not necessarily the aim of an organism, which is the focus of design. Many organisms can behave altruistically against their interest because of shared genes with those they interact with. In this way, survival has often been downplayed in Neo-Darwinian evolution, in favour of a gene’s eye view that makes replication the ultimate goal of the selfish gene. In regards to terms, I would prefer adaptationism, but I don’t know if that is just because it is the accepted term!
The reason why we use the word theory is because natural selection is not the only mechanism of evolution (which just means ‘change in time’ to many biologists who therefore include drift, mutation and so on as other ‘forces’ whose relative strength determines the trajectory of change) and because there were other proposed mechanisms of creative evolution, which the Gould 1976 article goes into some detail over. I could not add any further clarity but to say, again, that this is just the accepted word for a scientific hypothesis at the heart of a research agenda that is supported by evidence. I don’t know of any examples of mechanism of such scale that are not called theory (?).
I am not sure that I would argue that Creationism is a theory, because it does not conform to the scientific metaphysics of naturalism/materialism. To most scientists I know, I would think that Creationism was a priori flawed, i.e. before any evidence was gathered either way, because it relied upon tenants that conflict with the causal narrative that all science relies upon.
I hope that clarifies my view (as an evolutionary biologist) to you. I am not sure I can say anything more on this topic, but I would like to hear your response. Thanks again for engaging!