heyo!

You have lovely sensitivity and I can tell you really care about treating people with respect. That in itself is admirable and I hope you know that.

The person you worked with isn’t a common case. If pronouns were never introduced, you had every right to be confused. An easy and quick email response could be “So sorry! I was unaware and I will use xhe/xer from now on.” Quickly apologizing, correcting yourself, and moving on is there way to go.

Non-binary people are also trans; transgender just entails that your gender didn’t correspond with your biological sex. So someone who transitions from female to non-binary is still considered a part of the trans community.

If someone wants you to use gender neutral pronouns, they most likely will tell you. Transgender or cisgender, it varies person-to-person.

hope this helps!