OK, I have done some creative mode research. Based on my observations, Endercreepers do not target objects on the grief list that normal creepers and other types of special creepers do. Also, to address one of the proposed solutions, endermen and endercreepers WILL teleport through water blocks or water streams to get to valid teleport destinations, so encasing your base in water will do nothing to keep them out.
I have a couple of hypotheses as to how your base was compromised.
1) Invaders. These mobs are able to dig through blocks. They are personally chunkloaded, so you don't need to be at home. There are some defenses measures that can be taken to keep invaders at bay.
2) Vanilla Enderman griefing. Endermen could have teleported into your base and picked up blocks, compromising your defenses, and allowing other mobs to enter/spawn. This can be averted by using only two high ceilings or simply avoiding the use of enderman griefable blocks.
3) Ettin griefing. If an ettin manages to target you, they will break through "soft" blocks to get to you, including dirt. This may in turn compromise your defenses to other griefing mobs. It seems unlikely to me that the ettin could have targeted you in the scenario you described, however.
4) You missed a spawnable space when lighting up your base.
And, to address squitchy, creepers will not blow up quicksand simply due to being stuck -- only if they had targeted a player they then found themselves unable to reach, or if a griefable block is within their range. The other exception is doom creepers, which explode on death, without giving any damage to blocks. I am unsure if a dying doom creeper can trigger another type of creeper to explode and damage blocks.
EDIT:
An additional means of compromise, which probably isn't relevant here, is the possibility that an endercreeper can become aggroed by being looked at, which could possibly happen if you are AFKing in your base and one teleports in front of you. Also, I do not know if golems or other utilities mobs will target endercreepers, but if they do, that could also lead to an explosion.