Is it wrong to make assumptions if you're a scientist

You're making science out to be more complex than it is as a process. For example, light being a cosmological speed limit is a hypothesis. The simple reaction is to falsify it by providing evidence to the contrary. The process is simple, but achieving it is usually not.

This has a parallel to one of my favorite music quotes by Johann Sebastian Bach which goes something like: Music is simple, just hit the right note at the right time. The goal is simple, but achieving it may be anything from fairly easy to very difficult.

I'm not sure I'm on target for answering your actual question. To end on a slightly different note, scientists NEED to make assumptions to make progress. And equally, other scientists need to challenge those assumptions. The scientific method is fairly straightforward: observe, hypothesize (with assumptions if necessary), test, analyze and adjust, rinse and repeat.

You are correct.

However evidence does support the idea that light has a finite speed.

But as for dark energy accelerating the rate of expansion - that is, at best, pseudoscience. (my opinion)

However, you need to serve up to your lecturer what is in your course text books.

You can do that - then add a rider, that you believe there may be another explanation.

It depends on what your lecturer wants to hear. He (she) may be skeptical like you and I are. Or they may go strictly by the book.

And I concur...
It appears that there is an active policy at present in astronomy / cosmology, to dissuade research into theories that are competing with the status quo.