What a fun question about Duxbury. 🤓
I did notice something intriguing as I experimented with the first equation in your example under the heading "Recursive formula". If I added something that requires grade 1 mode close to the beginning of the symbols-sequence after the equals sign, then DBT will give us a grade 1 passage for the whole equation that you've enclosed in "ts" "te" codes (which give rise to the "math" character style). So, it looks like Duxbury's reasoning for using a grade 1 passage has something to do with how early on in the symbols-sequences grade 1 mode is called for.
So, I tricked DBT into thinking about that symbols-sequence after the equals as more than one symbols-sequence. (At least that's what I think I did.) I added a harmless code (xcs) after the r in the symbols-sequence "r, paren, a-sub-n-minus-one, paren"
Does that make sense?
I'm not sure that is a complete fix. Let's all keep tinkering with this.