If I understand correctly then the SA-1 can see the entire BW-RAM (mapping both 40-4F/60-6F spaces) but the SNES only sees half (the 40-4F portion)?
Right on that. I think your general understanding of the points you mentioned are also correct.
Certainly the SA-1 has great potential to hombrew if it could be fully leveraged. As you know, there is a pretty big learning curve to really being able to take advantage of this setup. It's of little surprise that most commercial games utilizing the SA-1 didn't even take full potential of it either. It's probably the most advanced and versatile of all of the add-on chips.