If determinism is true, would that mean that in a scientific theory, everything that is a theoretical possibility actually happens? I apologize if this question is similar to previous questions of mine.
If determinism is true it does not follow that everything that is logically possible happens. Determinism at least on one reading rules out any events other than those that actually (deterministically) occur. This does not exhaust the realm of logical possibility. It is logically possible that Donald Trump is a robot; the possibility is coherently conceivable = logically possible. This is so even though he is not actually a robot and if determinism is true, cannot be or have been anything other than he is, namely a human being.
Posssible worlds introduce another angle. In the actual world, or this actual world, if determinism is true then given the laws of nature and initial conditions, if X doesn't happen then X could not have happened and in this sense is not or was not a theoretical possibility. A modal realist about possible worlds can hold that there are other worlds, real and therefore possible, in which the laws of nature or initial conditions are different and an event, deterministically and theoretically impossible in our actual world, is possible and actually happens.
I think if determinism is true it means that everything is fact, the past, present and the future. We normally only associate fact with our observed history through memory of it. But Determinism means that we experience the eternal fact by the passage time we are subject to. If we were able to remember the future though our journey would be unnecessary as when we are brought into this world the passage of time would be unnecessary since our life of experience would be known if indeed things are determined and that fact is allready a determining factor. So I think what we experience is a function of memory and fact, and time simply allows us to experience the fact of our lives. Well that's my two pennies worth and humble opinion
"Is nothing a theoretical possibility unless it actually happens?"
Is this a question about what "theory" means? The philosophic meaning of theory is, very roughly speaking: orientation, something to meditate over concerning what the world is. From Plato up to the modern age. When the sciences took over the word theory, it began to mean something that can be applied. So, if one has, say, some reliable data about probabilities, say, actuarial tables, one can then apply them. There they give reliable predictions. The phrase "theoretical possibility" has to connect to a Sample Space where data points can occur during experimentation. So, if you want to say that Determinism means everything will happen, you must add to the conception of determinism an infinite time, or, whatever the conditions for everything to happen must be. That will be your Sample Space. In which all events are understood through the conception.