Lots of questions waiting for answers from the DOE, but in the meanwhile, here's some feedback on specifics that have come in via comments. (Thanks as ever for posting, and for voicing questions shared by scores of city parents.)
One parent asked about out-of-zone kids getting pre-K seats while her child wasn't seated at their zoned school. You can try to appeal the decision, but chances are, no one will officially tell you how the decision to exclude your child and include another was made. The party line is that no out-of-zone kids were offered seats unless in-zone kids were all covered; this may give you some basis for an appeal, but unfortunately, nothing's certain.
Bronx shrink, I checked the pre-K application again, and checked in with the DOE. The application does stipulate that the older sibling be continuing in elementary school -- but it doesn't say 'boo' about ranking the big-sib's school as a first choice. Waiting for official word back on this, and will report more when it's known.
Dr Monty asked about appealing middle school placement. Last year, a few districts offered appeals (details here); D 15 does, but D 21 and 22 do not. Brutal, I know. Insult to injury, the deadline last year for appeals was May 25th, well ahead of first-round news for most parents this year. Try contacting parent coordinators at the school/s in question to see if any informal process exists. Sometimes a bit of relationship-building can go a long way.
Keep the questions flowing.