Could you please clarify for me why the term "spare" and not "sibling" was used, if it's not possible to replace an heir if s/he dies?
A "spare" means more than one offspring from a couple—a non-immortal. The firstborn of the couple would be the heir and the targeted immortal. Any other offspring you would choose to have from that couple would be "spares." Or if you want to think of them as "extras" that's also accurate.
Am I correct in understanding that for the most part we will be primarily playing one sim and one first-born at a time, and that we will likely have a houseful of shiftless mooches? 
Not really. I guess it just depends on how deep you want to get involved with all of the characters. The spouses of the immortals could play a very key role and help out with the painting, ice sculptures, and photos of the immortals for the Museum. And of course, there's always cooking to do and repairing things which would be a good job for immortals' spouses.
Will there be a special sub-forum devoted to questions, stories, tips, etc.?
A whole new section? No. A dedicated thread in the challenge section, definitely.
Edit: Which Ambitions lots are allowed to be placed in Sunset Valley?
You cannot enter Edit Town, so unless you have Ambitions where a fresh neighborhood would automatically place those buildings, then you'd need to do without them.
"•All offspring must result from natural pregnancies of Sims living in the Dynasty house."
Does this mean that both parents must be living in the Dynasty house (other than the last generation, of course)? What if one of my female immortals has a pregnancy with someone not living in the house? Is that ok?
That's okay for the last generation, but aside from that, we're stressing cohesive family structure here—two parents living under one roof.
