Re# Tropical conditions: Don’t assume that ice crystals are restricted by geography; it’s more related to the airmass and conditions associated with very large storms.
Re instrumented engines: It is an expensive exercise as demonstrated by BAE/Honeywell late 1990s; BAe146 with instrumentation and camera in an unmodified engine alongside a modified engine to be exposed in the same conditions. This arrangement provided both a research base for the previously ‘unknown’ icing and a means of proving that the modifications worked given the difficulty in ensuring that the aircraft is actually in the ice crystal conditions relevant to the engine.