Buddhism is not a religion that exists in isolation. Sangha is one of the Triple Jewels right up there along side Buddha and Dharma (the Teachings). One of the things the movie seemed to really lack was the idea of community, community that might well have averted some of the more disastrous turns of events.
Mountain hermits may be romantic figures but they are not the ones who train novices normally and realistically some abbot or his representative should have visited from time to time and oversaw the running of the hermitage on the lake.
This wasn't really isolated. As we can see, the two girls visit the monastery in the beginning, and then the two police officers come, and then a mother with child later. Obviously the monastery is known about in the community. Maybe the community is also very small (we can't really tell)?
So the Sangha is there but just really small; it's just how it's manifested in this area.
I think if it were truly isolated then we wouldn't see the continuation of the temple through the generations.
Sangha in this context is the ordained community of monastics of which we only see two at a time. The young man who returns after his time in prison doesn't even have someone to over see his re-ordination and Buddhism is not a DIY religion. Monasteries run as communities and there is a complex system of control. Check out The Zen Monastic Experience by Robert E. Buswell, Jr. for more information.
We have no idea how isolated--or not--the men at the lake shrine were, because we only saw them at the lake shrine, and we only saw them on four occasions throughout their lives. However we do know that they made frequent journeys into the neighboring town for supplies. And we also know that there were many years that passed between the times that we saw them. It seems to me that there were, imaginably, many occasions and opportunities for interactions with other Buddhist clerics.
I am a Zen hermit. Beware of people who pound the "sangha" drum; this is often a weapon used to exploit those who would practice the Dharma. Buddhism is a religion of enlightenment, not of "community", or anything else. Activities that lead to that are "Buddhist"; those that don't, are not. If sangha gets you to enlightenment, awesome. If not, kill it. (That is, redefine it in effective terms.)
In the context of the film, the little boy is not a "novice". He's an orphan. Orphaned and unwanted children have long been left for temples and monasteries to raise in Asia. The monks do this out of compassion; their charges are not automatically considered monks-in-training.
Finally, I'm threatened by your dismissal of my entire practice as "romantic". Any damned abbot who comes in to my camp thinking to "oversee" me better be carrying a bigger stick than mine. I walk the Buddha's path, period. No time for sergeants.
Robin Rusty Ring: Reflections of an Old-Timey Hermit