Well, in terms of pronouns, if you really aren't sure you can always just avoid pronouns entirely. For example:

Elliot page is who acted in Juno. Elliot was pregnant in that film.

You can just use the name instead of their pronoun. It can be a little awkward, but if you are really concerned about not getting the wrong pronouns, that's the best option.

Normally I would just suggest using singular "they", but this can be problematic for some trans people who do not appreciate having "they" pronouns applied to them, especially when "they" is used when gender is perceived as ambiguous.

Imagine a binary trans woman who lives as a woman and wishes to be seen and referred to as a woman. If someone suddenly referred to her with "they", it might feel stigmatizing because it highlights how her gender appearance is ambiguous or not feminine enough to easily be referred to as "she".

So using a name instead can be a way to avoid this until you know for sure what their pronouns are (which you can do by checking in with them and asking, hopefully privately in a polite and affirming way that doesn't draw too much attention to their transness).

The problem is that trans people are so different from one another, so what is upsetting to one is affirming for another. There are non-binary trans folks who love having a gender that cannot be pinned down and who might find it extremely affirming to be referred to as "they" in the way I mentioned above can be upsetting to a binary trans person who is trying to pass as one gender or another.

Either way, my advice is to not get too concerned about playing the pronoun game perfectly, instead just center yourself in fundamentals: have the intention of respecting someone's self-identity, and learn to think about a person in terms of their identity to make that easier. If you are doing this, and it's clear you are doing this, a trans person generally won't mind a slip-up (they are probably used to honest mistakes and are only worried about people who are indicating they don't respect their identity, so just catching your own mistakes and correcting them quickly is more than sufficient usually).

Lastly, I did want to say one more thing about your question regarding Elliot Page: one reason to use the current name and pronouns when referring to a time before they transitioned is that it's not uncommon for a trans person to feel their identity did extend into the past, in the sense that they probably wanted to transition earlier and have always been that way.

I am a trans person who certainly felt this way, I distinctly remember being 5 years old and thinking there was a cosmic error and that I was meant to be born a girl. There is a feeling that I was sort of like a girl on the inside even before I transitioned, and referring to myself before as a woman or girl doesn't feel entirely wrong (it even feels like it honors a part of me that was not recognized or taken care of). But there are other times when I feel like my deadname or pronouns feel applicable, when describing a particular way I was before I transitioned. I did live as a boy and a man, at least socially - though always in an alienated and outsider fashion.

So it's complicated and probably very specific to each trans person how they think about themselves over time, but in general it's probably safe to refer to them using the name and pronouns they have now.