If creepers exploded due to simply despawning the world would be in utter ruins in no time flat; the average mob has an half-life of about a minute and the mob cap is 70, so every minute 70 mobs spawn and despawn, 4200 every hour, and about a quarter of those are creepers, so you can do the math.
Also, making mobs spawn more frequently would do nothing at all, except make mob farms more efficient - in my own mod I significantly reduced spawn rates as an optimization - but despite this I've been killing far more mobs than with the vanilla spawning. Why? Because the game is still capable of spawning more than a thousand mobs per second* - it is the mob cap that controls how many mobs there are, not the spawn rate - so the increase in density of mobs closer to the player easily offsets a lower spawn rate (I made them despawn more than 96 blocks away instead of 128).
*In vanilla mobs spawn within a 15x15 chunk area (225 chunks) around a player and each chunk is able to attempt to spawn a pack of mobs once per tick, 20 times per second, and each pack has up to 12 individual spawn attempts (3 for each of 4 mobs) - that's a total of 54000 attempts per second. I modified this so a circular radius of 6 chunks (121 chunks) spawns mobs with a quarter of chunks spawning mobs per cycle for 7260 attempts per second, and if the mob cap is less than half filled the number of individual spawn attempts is reduced to 4 for a total of 2420 attempts per second - this is a worst-case reduction of more than 95% yet the mob cap still fills up within a couple seconds. Indeed, on an empty Superflat world this cost the majority of the server tick time - which fell from 3-4 ms to around 0.2 ms. Memory allocation rates also significantly dropped (the game creates a new entity for each spawn attempt, even if it isn't successful). This change is also a direct nerf to mob farms, along with mobs only despawning based on horizontal distance, not vertical distance.
Some of the other things are just as bad as exploding creepers and serve no purpose to add real difficulty to the game; infinite fire spread? One randomly generated lava lake and it is all over...
Also, the very definition of "hardcore mode" is usually permadeath, i.e. one life with no respawning, the same definition used in Minecraft, so allowing the player to respawn would no longer make it "hardcore" by definition.
*You can only do damage with your weapon when your weapon has cooled down.
- then.. whats the point of pointing a sword or your hand to your enemy?
The only difference this would make is that you can't attack at all until the full cooldown period has elapsed - either all or none, instead of all or reduced damage. In fact, this may actually be a feature in an upcoming version:
Players can now only attack when the weapon is fully charged.