[David Foy's-- can you suggest how Foye (as in foyer) would become Feÿ (as in pie)? Is it likely that these Feÿs were originally French Protestants (Huguenots)? Or is it more likely that Feÿ was originally a German name?]
Perhaps the better question.....,my dear friend, is how did Feÿ became Foy?
First, some clarifications: 1- The dieresis or dot-dot over a vowel as in ÿ is not a notation peculiar to German; it is quite normative to FRENCH so when explaining pronunciation of ÿ, I would proffer that it is best to consult a French expert rather than a German (perhaps). 2- Based upon your own citations of Wilhelm Feÿ and company, it is clear, that in the case of this family, Feÿ precedes what became Foy as in your Simon.
Based upon above, I will presuppose that the root Feÿ precedes Foy for the discussion of the German line.
Since there are several villages in France with the name (or part of the name) having Feÿ, and, because these villages are predominantly in the Alsace, Lorraine area, I am not going to immediately jump to the conclusion that the Feÿ line is French. Step-by-step, I should like to investigate the background of Feÿ. Punct Final!
French locations with Feÿ:
Fey-en-Haye (six miles NW of Nancy and 10 miles southwest of Metz)smack in the middle of LORRAINE or (German France).
Fey (5 mi SW of Metz)LORRAINE [Fey,FR65,FR]
Fey (10mi SW of Metz)LORRAINE [Fey,FR62,FR]
Fey (40 miles SW of Lyon) [Fey,FR36,FR]
Feydel (30 miles SE of Lyon)[ Feydel,FR48,FR]
Feyssaguet [Feyss aguet] [FR21, FR] (watch, or vantage point)140 miles West of Lyon
It is interesting to note that the Chateaux de Fey is located 35 miles SW of Troyes in Champagne. Château du Feÿ, located near Joigny (between Sens and Auxerre - 95 miles south of Paris) is a splendid property classified "monument historique" dating from the 17th century and is lovingly restored. The property stands on 100 acres overlooking the Yonne river valley with superb views over the countryside. Not far away are some of the wine areas of Burgundy, Champagne, and the Loire, with Paris only 1 1/2 hour's distance by car or train.
All of the above suggests that, dating from the 1600's someone named Feÿ had a pretty nice estate 1/3 of the way from Paris to Lyon. If there is a connection between the different geographical Feÿ locations cited above, it might mean that as late as the 1600's somebody named Feÿ who lived in the Chataux had his influence felt all the way to the borders of Lorraine encompassing Metz and Nancy and perhaps Alsace and as far south as Lyon on the South east and Conques on the Southwest. The primary land holdings encompassed are, of course, wine. bounded by the Rhone River and Saone and the Rhine tributary on the north and east.
It seems quite plausible then that anyone plying his trade in the Lorraine and Alsace areas, and perchance, also Champagne and Burgundy, might very well have moved his trade up the Rhine if he had some problems.
Many of my Merkel side of the family were barrel makers. Obviously, this trade was up the Rhine from Lorraine and into the Odenwald from Heidelberg to the North including the Pfalz (Neustadt). That these trades existed (Kufrei) or Coopers and Binders or Benders in this region in the 1500's and 1600's is easily documented.
Perhaps our Feÿ families were Franks who settled in France and were given some land for their service under the aegis of Charles Martel and descendants and amalgamated into France.