I think the question is further complicated by whether a population is literate or not. Thus, French, has spun of many pidgins, but the fact a standard base of French literature exists, serves to keep the entire concept of french as a language from splitting.
Thus, although Haitian is a strong and vital creole form of French, the chance that it would spin of into another language is relatively small, because as indiviuals become more literate, they tend to adopt the standard forms. The power of literacy can be easily seen by comparing two different languages from 2000 years ago, Greek and West German (Proto-English). Greek has undergone no where near the number of changes that have occurred within the English family tree.
Thus, with respect to your question, would OTL English be understandable by an ATL Mithraic Anglisch? Probably not, particularly if the langauage is frozen early by literacy.