@@ -14 +14 @@
-<gloss g_type="expl">person below the department head with responsibility for a particular area]</gloss>
+<gloss g_type="expl">person below the department head with responsibility for a particular area</gloss>

Ngrams (63k), Eijiro, Denshi.
http://ja.wikipedia.org/wiki/部長 : "また、地方公共団体によっては部長と同格の幹部であるが、担当する部を持たないポストを「担当部長」と呼称して特定の重要事務を分掌させているものがある。"

May be a simple A+B, but I think there is enough of a nuance to its meaning to keep it.

@@ -12 +12,3 @@
-<gloss>senior Manager</gloss>
+<gloss>senior manager</gloss>
+<gloss>director</gloss>
+<gloss g_type="expl">person below the department head with responsibility for a particular area]</gloss>

I don't think 担当部長 deserves an entry in a dictionary.
It's not in any of KOD's dictionaries.
I think it should be seen as two terms that just happen to be sometimes collocated.
e.g. ニューメディア担当部長 would be a "head of new media"
-- literally, a department head (部長) in charge of (担当) new media (ニューメディア)
.
If it is retained, however, there is no reason for "Manager" to be capitalized.
But I don't think 担当部長 means "senior manager".
Rather, I would say
...担当部長 = head of ...
which is difficult to convey using EDICT's format, I think.