OK, @Florian_May you got me on the complexity 
Here's a schematic of my situation which I've given to enumerators, which is
a) much less involved
b) not as well illustrated
A-D are the main groups, but I haven't tried using nested repeats within this as so far it's mutually exclusive - one set of data for a given location. We're looking a peat condition so fewer parameters (no more than 6 for each location).
However, I have found in the field that things are pretty robust - typically collecting 80 - 100 sets of data within the single form instance. There have been cases where Collect has crashed (sometimes on the Geopoint widget using Mapbox with some 'null value' message) or other times where we're using another app in parallel (e.g. clinometer or gps) and Collect restarts mysteriously. So far, I've not lost any data - on restart, selecting 'fill blank form' comes back to the saved version of the form in 'jump screen' and we just need to navigate to the right 'level'. I've also found it easy to save and exit the form during a break (e.g. caffeine injection) and 'edit saved form' to restart data collection where I left off. I've also been able to use Mapbox layers to load my own geo-data to help navigate to specific locations for my geopoints, which helps 'package' things into a single app rather than needing to switch constantly between apps.
The improvements to the navigation / jump screen (which I think you contributed to!) have made it much more viable to combine forms - it's pretty close to 'intuitive' now, just need to get that 'Add Group' message changed! Drilling into nested groups needs some work (I use 'dynamic' group names based on answers within the group, so that helps), but I can see that new enumerators could easily get lost in your forms! Maybe some kind of colour coding for different nested levels might make it easier?
One point, I've moved away from making questions 'required' as it can be frustrating if you miss one and then have to navigate back to some nested question (because the jump screen lets you bypass required questions, so is easily abused!), but if you just had top level 'required', such as the enumerator name, that might still work.
Looking at what you're collecting, I reckon your volunteers need way more than a 'well done for surveying for XX hours' message . You've obviously got some pretty dedicated folk helping you.
Anyway, keep up the good work...