Regarding three, one dead accurate indicator is, IMHO, when people that voiced their opinions and were higjly engaged start to become quiet. This absence of "distracting noise" is sometimes even seen as a sign of improvement, in reality it is the beginning of the end. And so hard to catch, since us humans are bad at realizing things aren't there anymore.
1) losing a critical mass of developers is the beginning of a death spiral
2) the critical mass is smaller than you think
3) you lose the developer months before you know you’ve lost the developer