Dumat-Mythal
Lusacan- Falon'Din
Toth-Sylaise
Razikale-Dirthamen
Zazikel-(Chaos)
Urthemeil-(Beauty)
Andoral-(Slaves)
Possible 8th unknown god- "Draconis"-
Now we have left June, Elgar'nan, Andruil, Fen'harel and Gilain'ain.
June=Craft
Elgar'nan=Vengeance
Andruil=Hunt (Sacrifice)
Fen'Harel=Rebellion
Gilain'ain=Halla
I think we can leave Gilain'ain off the list because she is not original. She was made. I also personally think she was insane and may have created the Qunari out of dragons and elven slaves but that's another theory.
I'm less sure of the others. A case could be made for either Andruil or Fen'Harel to be Andoral. She liked slaves, he (may have) led a rebellion of them.
June could be Urthemeil, if you connect crafting to beauty, I suppose.
I think you could even make an argument that Dumat is Elgar'nan and the 8th, Draconis, is Mythal, given her dragon form.
I don't know! 
We could leave Fen'Harel off, make Zazikel = Elgar'nan, Andruil = Andoral and June = Urthemeil.
Still...
I definitely agree about Toth = Sylaise, both are associated with fire, so this would be the most simple connection. I have also considered Lusacan = Falon'Din, as night is the time you dream, and is somewhat related to death or the fade, though, this is a more loose connection. Another possibility is that Lusacan = Elgar'nan, because of the association with night, and how, in his legend, Elgar'nan 'defeated the sun and cast a shadow over the land', aka it was always night, until Mythal convinced him to make a truce.
I think it is less likely for Fen'Harel to be Zazikel or Andoral, as they were slayed in the second and fourth blights, and (so far as we know) were not made into Old God Babies, so likely perished. What I think is more likely is that Zazikel is a possibility for Elgar'nan as well. He's known to have quite a temper, and for being a little unstable, so chaos would fit. This would also have made him rise secondly after Mythal, fitting for an all-father. Other possibilities are Falon'Din, Ghilan'nain, or Andruil. Falon'Din for the wars he caused, Ghilan'nain because of the terror her creatures apparently brought, and Andruil for the codex we got about her descents into madness. We know so little about June, so I suppose he could fit as well.
Andoral I believe would best fit either Andruil, June, or Falon'Din as well. Andruil because she supposedly sacrificed her people (maybe blood magic?) I'm guessing this would require slaves. There was also a codex entry for her in the Temple of Mythal of her people pleading to her, much in the way slaves would, (or something like that, I can't quite remember.) Plus, the similarity in the name, I suppose. Falon'Din, again for the wars he began to gather more worshipers, so maybe worshipers = slaves? And again, June, because we know so little. Poor June.
And Urthemiel, I believe could either be June or Ghilan'nain. June, for the reason you stated, or Ghilan'nain for that Hallas are often described as beautiful creatures, and I believe her legend states her as especially beautiful, but I am not sure on that.
If it wasn't for the association between Mythal and Dumat, I would have thought that Dirthamen would be the God of Silence, relating to secrets fairly well. Though, this would have made him the head of the Gods, and I agree that that position is better reserved for Mythal. So, I believe that Dirthamen = Razikale is the next most likely connection. Something else that crossed my mind, is that perhaps Dirthamen and Falon'din are one Old God, instead of two. The two are described as brothers, but closer than any label that could be attached, not lovers, but also referred to as 'twin souls.' This could mean that they are the same being, or perhaps they are one Old God soul, split into two forms when they are elves, but turn into one Dragon. It could also be that one is the body vessel for another. In the same way that Flemeth is a vessel for Mythal, but both beings are present as somewhat separate entities from each other, and share a form.
Another possibility (and sorry for all of these) is that Falon'Din is, I guess you would say, 'possessing' Dirthamen in some way. Think of the possession cases we've had in the game so far, such as Connor. He and the demon were one entity on the physical side of the world, but when the player enters the fade, we can see both Connor and the demon as separate entities. So perhaps Falon'Din is able to exist separately from Dirthamen while in the fade, but 'possesses' Dirthamen in the physical world, where they become only one Dragon.
If these two do somehow make up one God, with any of these theories, then, if you include the 'scraped from record' eighth god, there are enough gods for each of the Pantheon to correspond to one. I'd like to believe that any of these solutions are possible. And then, of course, there is the possibility that they are just two separate Old Gods, and that one of the Pantheon is not an Old God (likely Solas or Ghilan'nain, as you said, as they are the outliers.)
So, with this, I would say:
Dumat (Silence)- Mythal
Toth (Fire) - Sylaise
Lusacan (Night) - Falon'Din or Elgar'nan
Razikale (Mystery) - Dirthamen or both Dirthamen and Falon'Din
Zazikel (Chaos) - Elgar'nan, June, Falon'Din, Ghilan'nain or Andruil
Andoral (Slaves) - Andruil, June, or Falon' Din
Urthemiel (Beauty) - Ghilan'nain or June
And lastly, is the eighth 'unkown' God, I think this God is most likely to be Fen'Harel. Or perhaps Elgar'nan, struck from the record for a possible betrayal of Mythal (husband betraying wife seems to be a common theme with this game.) Like you said, it may even be Mythal, but then this would throw the whole Dumat = Mythal theory. But, I think most likely it is Fen'Harel. I'm going to load up my game and re-read that astrarium's Codex, as I cant quite remember what it was. And then I shall craft a wonderfully long tin foil-hat theory that none shall bother to read. (Again, really sorry for the long post, I do hope someone reads this though, and can either add to or challenge some of these theories.) Mostly, this is all just pure speculation, nothing is certain until game-confirmed, after all, but I love any and all theory crafting either way, and really hope this thread isn't lost.