Nope, just like "(ε)βδομάδα", but it's unlikely to find these versions with the omitted vowels in a formal text ;)
Is there any real difference between επόμενος and ερχόμενος or are they just variants of the same word?
I don't know, but when looking around on the internet I see a lot of Την επόμενη μέρα. I think maybe ερχόμενος can be used for phrases such as Την ερχόμενη κυριακή but that την επόμενη Κυριακή is more common? Maybe we'll hear from a native Greek speaker. I wonder if επόμενος is more common because ερχόμενος is a participle. Here's a wiki link that gives an example of the participle in use with the sense "forthcoming": https://en.wiktionary.org/wiki/%CE%B5%CF%81%CF%87%CF%8C%CE%BC%CE%B5%CE%BD%CE%BF%CF%82
Ερχόμενος and επόμενος are not the same. Ερχόμενος means the one that's coming and επόμενος means the next one. So, if it's Friday η ερχόμενη Κυριακή is in two days, but η επόμενη Κυριακή is in nine days.