The German Wikipedia has a short article on the usage of "ey" instead of "ei".
What it says is that even until the early 19th century it was quite common to use "ey" or "eÿ" instead of "ei". This originated from the earlier spelling "eij". The "j" in "eij" was originally a different way of using "i", which could be used at the beginning or end of a word.
I do not know if "ey" was pronounced differently than today's "ei", I can think of a way that it could have been but I am not sure at all.
Regarding spelling reforms, the same article references these as well. Pretty much what it came down to was that there was an "Orthographical Conference" in 1876, first rule books came out in 1879 and later, but they were not well accepted by the public and in general the rules were implemented quite differently in the different German states.
In 1901 the German written language got its first uniform rule set. The "Duden" became the official dictionary and in 1903 the new rules were officially used in administration and so forth.