Does the answer you gave re earliest maariv apply to davening alone as well as with a minyan? I had understood that early maariv (before dusk or nightfall) applied only if davening with a minyan. Am I mistaken? Also, is there a concept of "dusk" before "nightfall", and how does that enter in to this question? Thank you!

The earliest time for Maariv is from Plag Hamincha. (See Shulchan Aruch Orach Chaim 233, 261, 267) but this would require repeating krait Shema after Tzet Hakochavim. . (See Shulchan Aruch Orach Chaim 235). The Shulchan Aruch does not differentiate if one wishes to daven as individual or if one wishes to do so with a minyan. However, if one davened mincha after plag hamincha and then decided to Daven ma'ariv also before Tzet hakochavim,on erev Shabbat, as an individual he should not do so because his actions are contradictory. Either it is day since he davened mincha or it is night and Shabbat since he davened Maariv. However, with a minyan according to the Mishna Brura (267:3) even if a person davened mincha after plag he may daven Maariv with a minyan before Tzet Hacochavim. However, the Mishna Brura does stipulate then one may do so only in extreme situations and if it is already "Ben Hashmashot= dusk. This would already answer part of your second question in regard to the issue of the ramifications of dusk. It would be difficult to relate to all its ramifications. Therefore, I suggest that you ask again when a particular situation arises.