Having established that a shared space was, indeed, successfully enabled and used what was the likely degree of freedom to carve? At minimum there must have been tolerance over a wide time period – an acceptance of different and changing beliefs. Perhaps this tolerance was exploited, however, to influence transition between different belief systems.
Full independence to carve at will seems unlikely – the space would, most likely, not then have been amicably shared and the result would be our seeing less than intact surfaces. This is particularly applicable when Christian Crosses appeared on Stones with Mithraic objects.
Could portrayals of beliefs (and secular carvings) have been influenced, over a long period, by some sort of constraint, rules or bounds? Conversely, maybe there was some implicit respect (for whatever reason) that enabled this freedom for the carvers and especially for those who pursued or practiced the beliefs.
Looking back to Roman times there was clearly an acceptance or tolerance of different beliefs with iconography from several beliefs within Mithraic temples. Taking the London Mithraeum, for example, from the layout of the space, the tauroctony marble and the Mithras head, the temple was created specifically for the purpose of pursuing Mithraic belief. However, membership of the Mithras cult seems to have been compatible (at minimum, not incompatible) with worship of other Roman gods of the time – sculptures of Mars, Mercury and Minerva were found when the temple was excavated. Maybe there was an intrinsic tolerance within the Mithraic following which showed itself in Roman times and later when Christianity appeared.