Jenny,

Re Viola sororia/papilionacea: see link below.

Re V. sororia/cucullata: cucullata should stick to wetter sites whereas sororia can occur in wet or much drier spots. But that and issues of hairiness are all weak discriminators bewteen several species and infraspecific taxa, partly because they vary geographically, but hairiness also varies within a single plant during a season. Later in the year, features of the cleistogamous seed pods and sepals distinguish these species, but it is not so easy when they are in flower - or at least it is not so easy to describe. The one factor worth considering, though again it is a little imprecise, is how far above the leaves the flowers are held: V. cucullata holds them more or less at the same level as the top of the foliage, but V. sororia holds them above the foliage.

Thanks for your good description of the associated flora; in this case it does not exactly answer your question, but the forest aspect suggests V. sororia more than V. cucullata.

Must have been a great sight,
Mike

Here is a link that might be useful: Prior discussion on Viola sororia/papilionacea