Solas romance gets less screen time compared to others, but Cullen? Correct me if I'm wrong, but I've done/seen other romances aside from those two and it didn't seem like he got less content than the rest.
Also (out of curiosity) - is it confirmed that it was entirely new, later addition, or is it something that they might have worked on initially, got cut away and then re-added during additional development year?
There was no mention that it was cut and re-added. Unless my memory is mistaken (and it could be,) Cullen was announced as a LI not too long after Cassandra was back in spring/summer of last year, though their sexuality wasn't revealed until later. I think we can safely bet that Cullen was intended from the get-go. I don't think he was added as a LI later in development. For the record, I've done both Cullen's romance and Iron Bull's romance. I personally saw no difference in the amount of content, and I was one of the loudest critics of NPC romances for awhile there.
As far Solas, my guess (and it's only that) was he was added sometime after the first delay. The game was initially supposed to come out in 2013, but EA got smart and gave the BW team an extra year. Originally, the game was supposed to be human only with Iron Bull as a gated romance. When they were allotted the extra time, that's when we got the multiple races back.
I also know that it was only due to John Elper (Lead Animator) working some crazy overtime that is the reason why Bull isn't gated. I read the twitter thread during that time, and apparently 16 hour days weren't uncommon. So yeah, dwarven characters would have had only Blackwall to choose from if they hadn't worked so hard on IB prior to the year extension. As it is, bi/straight women characters do get two extra romances, that are gated. Elven ladies get four, then human ladies get three with the other races at two. I'm starting to get better about dwelling on the numbers, but since it does come up, those numbers come with a few caveats.
I think if straight male characters get a third romance in the next game, I think it should be race gated. As I said earlier, in this case it isn't a matter of what's fair so much as what's interesting. This doesn't apply to just straight male characters, but I have noticed that a race gated romance can have a very good story in it. Like having a lady Vashoth LI gated and open to dwarves and Qunari, and I think there could be a neat story arc with a Tal-Vashoth/Qun/Vashoth player character.
That's just my take on it anyway.