In my experience becoming non-verbal may also depend on who you’re with. It might be that your partner goes non-verbal with other people first and you last (since you are the most accepting of this).
If you’re able to speak for your partner in those situations (I suggest discussing how they should signal to you that this is needed, plus in what situations it would be appropriate) that can be a big help.
Also, as a couple of other commenters have mentioned, look into other forms of communication. Texting or email is often still possible if not too overwhelmed, so again discuss with your partner - perhaps there is a shorthand set of messages or emojis they can use when they’re struggling to communicate that will allow them to convey what’s happening and what they need.
Yes! I have to understand how a process works before I can do it myself.
To all the people who just “give it a go” without knowing exactly what they’re doing - I cannot comprehend how you do that.