2.) linux rarely if ever broke ABI in the same major release and revisions [3.2 3.2.1 3.2.3] and since some time ago LTS releases are ABI stable very long time
3.) if you mean you have a 3rd party BLOB driver like FGLRX or Nvidia's and you want to run it outside the supported kernel versions for that driver is your fault not linux but if you use a linux supported[in tree] driver this rarely even happens and this happens in almost any OS including windows [vista driver in windows 7 == missing features - crashes - pain] you just think windows kernel is ABI stable cuz it has longer release cycles and you think since windows vista and 7 are different OSes is 100% rational to download new drivers[but no NT kernel 6.0/6.1]. So in resume linux major release cycle is 6-10 weeks windows is 2-5 years that is all, so a 3rd party driver need to stick with an LTS kernel release or put resources in support every major version each release cycle[if linux releases a major every 2 years you won't even notice this <-- this is what LTS is for].
3.a) we can discuss that maybe the linux release cycle should be extended or maybe provide an isolation layer between drivers and kernel API or another solution but a 3rd party is not responsability of linux[as was not microsofts the vista nvidia drivers mess] so the risk of breakage is always bigger than an linux natives in-tree drivers
3.b) internal kernel api changes breaking userspace is a rare scenario too and if it does normally in the changelog they put a big fat warning that an userspace software requires an upgrade to X.x.x version [kernel ppl are not crazy ....]
4.) about POSIX please post some code example and some logic behind cuz well i really can't see it useselfuness and looking in msdn that feature is deprecated for windows too[at least under suspend]
5.) about ada can't comment since its outside my area of expertise