My Amell's story is quite over for me, and she got Leliana with her back then. I prefer her died as a hero, turning a new page for Cullen since now he knows he will never have her. (Guess that I'm in the mood of having sad stories
)
Weirdly the first time I got interested in Cullen it's in DA2, starting with "hey I know this guy!". And got crushed to see that he's not romanceable there. But then Hawke got Fenris/Anders/Isabela/Merrill so no big deal.
Oh, since I know that could be sort of problematic,
I won't take my chances... not in my first PT anyway. My first self-insert PT in DAO was with a female Cousland, and that's what I'm gonna pick for my first DAI PT. No troubles with the Amells at all.
Btw, I have my moments too, for sad stories. 
Weirdly enough, I got interested in Cullen the same way you did.
The difference is that I was already interested in Anders, and I happened to play a female Rogue (also self-insert) who left Bethany in Kirkwall, to be safe. So, when Cullen arrested Bethany and took her to the Circle, I was very angry with him for a while.
......until I realized how much I actually liked him.
Later, I would go back to DAO, and install mods and play the mage origin. My first mage origin was with a female elf (also self-insert
). Then, in my second DA2 PT, I planned to Rivalmance Anders (Friendmanced him in the first PT). But now I was crazy about Cullen, and I would spend hours snapping screenshots with him and my Hawke. At times, I would remind myself: "hey, you're romancing Anders! taker your eyes off Cullen!"