We have only one BC and are currently in the approval process. We have no (known) medical reasons for not having more BC so maybe similar in that way.
The 'adoption by choice' thing never really has been a problem in the approval process so far. We were asked, of course, why we chose adoption, but our answers always seemed to be convincing enough to take us further.
Regarding the matching considerations, we are obviously always thinking 'what would this mean for DS', so whereas we are quite open to 'mild' versions of many things, 'moderate to severe' versions of equally many things do make us tick the 'no' box. (E.g. hearing impairment that needs hearing aid ok, but deafness and having to communicate by sign and needing special schools, not ok). This means that we are only really willing/able to adopt a child who has relatively minor issues, and is thus 'desirable'. A couple of SWs told us early on, that a few years ago they would not have agreed to assess us, as they would want to 'keep' these desirable children for childless couples who were turning to adoption as a last resort, whereas we already had a child so were less 'needy'. However things seem to have changed in recent years and they do now assess many people, even if they are after a healthy white baby with few issues, and have BC already.
That said, the reality is that 'issues' are part and parcel of adopting, and the choosier you are, the longer you may have to wait for a match.
In reply to 4) this is a question we ask ourselves too. I think it is important to recognise that you are having another child (be it by birth or by adoption) for your own sake, not in some way for the sake of your child(ren) (as in, providing a sibling for them or such). We do weigh how a new child's needs would affect our existing child, and would say no if we would project that an AC would significantly limit his life opportunities. However, a child having special/additional needs, can mean just the opposite: It can provide opportunities to grow emotionally, make you stronger, make you more tolerant and empathic and accepting of difference, so I'm hoping that we can achieve the right kind of mindset to see things in this way, rather than focusing on the limitations and restrictions.