Thanks @wroos for this deep review 
and your precious advises !
Yes, mapping into repeats works fine. The only limitation is that collect will only show on the instances map the first geopoint of the form (outside the repeat).
About the complexity, such multi-level (2) repeats form is quite often use in species monitoring, depending on the applied protocol. The first showcase you mention uses 2 repeats, one for the locations and one for the species, but it does not include the calculated location for each species (we manage it in our database)