An idea to solve your problem with your cells is to make the cell walls out of Obsidian or Bedrock. these should be enough to not be destroyed from the explosion of a Creeper.
This however leave the door, while I can't be say for certain if an Iron Door will be enough you might want to use Lava.
To do this, create a 3x3 square with the middle block empty, this will be a funnel to direct the lava, one you have it facing down the lava will fall straight down, make sure that where it lands you have the block missing so that rather than spreading into the cell it'll just stop.
This has 2 advantages
- The escapee will get damage as they walk though
- The escapee will be on fire taking damage slowly for a little while
Both will make them weaker to the creeper's explosion.
Now to have the Lava Fall act as a door, at the mouth of the funnel set up a Sticky Piston move a block in front of the mouth to stop the lava and move it back to reopen it
Lockpicks, under this system, you could use Redstone and have a second Redstone circuit which is broken and if fixed will active the Sticky Piston and block the lava flow, though I have no idea how to do that. and of course you'll need to set up a "window" in the cell to avoid the redstone from being drawn though the lava destroying it (a block on the floor missing so it can be drawn though it rather than the lava door entrance).
Getting Creepers to drop something other than gunpowder you'd probably need a mod or mod the game yourself, same goes for increasing collecting distance.