Now I have to assume (but correct me if I'm wrong) that a Valkyrie should be about equal in Refresh to a senior council member.
Not necessarily. The only Valkyrie we've met thus far has capabilities roughly equivalent to Harry's (see Herot, where the two are on roughly equal footing). Changes Spoiler: When we run into Odin, he says that his powers have been diminished in recent centuries, so it would stand to reason that his Valkyries came down in the world with him. This could explain the seeming discrepancy betwen myth-Valkyries and DF-Valkyries.
Odin should be very much beyond any mortal refresh, so what might that look like?
There's really no way to tell, we haven't seen a god throw down with a worthy opponent. The stats in OW are pretty much pure speculation, and since Odin is a plot device anyway you should just give him whatever powers you deem necessary and thematic.
Finally - if the "gods" have a refresh equal to Odin's level (roughly) - what would the elder gods (Titans) look like, refresh wise?
Well, there's no real way to know. If DF gods rely on mortal worship for some or all of their power the Titans would probably be in even worse shape than the gods that overthrew them.
So we have the Norse Myths represented in the books - what about the Greek or Egyptian myths?
The greek gods are represented. (Welcome to the jungle and Dead Beat spoilers): In "Welcome to the Jungle" (the graphic novel prequel to Storm Front, written by Butcher) Harry faces off against Hecatean Hags (a cult of hags that worship Hecate) who are seeking to perform an ascension ritual (identical to the ritual from Dead Beat). When Harry asks Bob what the ritual would do, Bob replies, "Where do you think Hecate came from?"
I've always suspected that the reason the other pantheons don't feature more prominently is that Dresden (and probably Butcher through Dresden) willingly admits that his expertise leans heavily toward western European folklore. Hence, we see a lot of Faeries, Norse monster, and Arthurian references, and fewer Greek, Hindu, Mesopotamian, etc.
Why would valkyries be at Senior Council level?
Valkyries, as the legends portray them should be about that powerful. but Dresden Valkyries don't really work that way. The stories, most heavily "Aftermath" imply that Valkyries are just normal women, who are imbued with power by Odin. Basically, if you're female, and have sponsored magic: Runes, you're a Valkyrie.