Protection against Instant Death is a minimal, minimal change that doesn't fix the Codex in the slightest. It makes Warriors ok, but not great, and is pretty much useless for everyone else.
If I were to make a change, I'd revert Instinctive Behaviour but remove the Leadership check; if you're in Synapse, the Hive Mind controls you, you're Fearless. If you're outside Synapse, you get Stealth/Rage depending on who you are, but you're not linked up to the Hive and you go down to Leadership 5. So yeah, you can move dudes out of Synapse to get buffs, but you get a big debuff to counterbalance it, rather than just have a single big debuff.
That's what I'd do, if I changed anything.