Non-Selected Repeat Group Questions

1. What is the problem? Be very detailed.
HI All,
I'm about a month into my ODK experience - can manage most of my needs but have hit a stumbling block.
Part of my form is attached: here i am asking the same series of questions about different fruit crops. Each crop (mango and lychee shown here) has specific pests and i'm asking questions about the abundance of each pest. The select_multiple lists the pests of most interest, once this is done there is an option to add as many other pests as required via a repeat group.
The issue i can't resolve is that i do want the non-selected pests of interest to be given a default 'Abundance' of "N" (since absence data for important species is important to capture). I know i need to do this from outside the repeat group, perhaps via a not-function or a != function but the problem is way out of my league.
Equally, some insects feed internally so some of the questions are irrelevant (presence of adults/juveniles etc) so, ideally, i only want to show the relevant questions for each chosen pest.
My lychee group shows all questions for all chosen pests. My mango group is an attempt to only show relevant questions for pests that are internal 'hidden'.
It is a clunky solution - any advice towards a more elegant approach is appreciated. fruitpests.xlsx (44.7 KB)

2. What app or server are you using and on what device and operating system? Include version numbers.
ODK Collect v1.22.4 on a Moto android phone. Using Briefcase to pull and export data.

3. What you have you tried to fix the problem?
i have scanned the forum trying to add lines to the form - but i tend just to get confused, not enlightened.

4. What steps can we take to reproduce the problem?

5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.