Zevran and Leliana [edit: and Isabela], as far as I know, are the only definitively bi characters in your list. Everyone else is subject to interpretation/debatable at best. Unless I'm missing something. For instance, has Celine or Marjolaine had any expressed inclination toward men? They could very well be gay for all I know.
The bigger issue with your first point is the keyword possible. Yeah, anything's possible if it isn't defined one way or another. That's why, when people cite what's possible, rather than what's concretely defined, as a reason why one thing should be this way or shouldn't be that way, I see it as a cop-out.
As for your second point. I feel like it's sort of addressed by dwarf and elf cultures. Considering their numbers and fertility plights, if I was an elf or dwarf with inclinations that wasn't conducive to solving their plight, I would be inclined to keep those inclinations to myself for fear of being labeled a traiter. This is established lore. Which brings me to the question: how many among those who site "Well it's Thetas" will happily play an elf or dwarf to romance, say Vivienne, without regard for how an actual dwarf or elf may approach this?
To your first point, Anders and Marjolaine are both confirmed bisexual. Anders has multiple dialogues in which he pretty much explicitly states that he's attracted to a person, not a body and Gaider has confirmed that he was conceived as a bisexual character back in DA: A development. Marjolaine is shown seducing at least two different men in Leliana's Song. Celene might be a lesbian, sure. But I was really talking about LGBT individuals, not just bisexual. If we expand the list of known LGBT individuals, it gets even bigger: Maevaris (trans-woman), Nicolas and Julien (gay men), Wade and Herren (confirmed as a couple), Serendipity (trans-woman), etc. My point was that there are a lot of openly LGBT individuals in Thedas, so I think it seems silly to think that we can't use their existence to prove that it's not uncommon in that world.
To your second point, it's absolutely not anywhere in the lore that dwarves and elves are less likely to be gay because of their small numbers. It states that there is pressure to still reproduce. The act of reproducing doesn't mean that you aren't gay anymore. I could out and sleep with a woman and get her pregnant because I want to have a kid, but it doesn't mean that it fundamentally changes my hard-wiring or orientation. For example, my dwarf noble did just that. He slept with Mardy to have a child so that his father would get off of his back about producing an heir, then he (grudgingly, as he was the only choice) went off to have a romance with Zevran. That doesn't make him straight. He's still gay because that's how I "created' the character. He had a kid out of duty and was then free to have a gay relationship with his "love" Zevran.
My point is that I don't see anything at all that tells us that we have to stick with a 4-10% LGBT representation just because that's how it is in "real life" and, if they chose not to do so, I don't see why that is any more unrealistic than a fire-breathing dragon, demons, dwarves, magic, etc.