Andy,
I would suggest briquettes. I highly doubt you will notice any difference in flavor by distributing the lump into the unlit. You may not even notice anything by switching it all to lump.
The main benefit in lump, in my opinion, is dramatically less amount of ash it will produce. Briquettes have binders in them to hold them together, which create more of the ash. But briquettes are packed tight, so they burn even and consistent.
Water provides a heat sink that will prevent high temp spikes as water can not get over 212 degrees. However, it absorbs a lot of energy to maintain that temp, eating up your charcoal.
For overnights, I always packed in a lot of briquettes, Minioned the start, and used water. I have just recently switched to a foiled pan on overnights. If you can start your cook early and watch it initially, then you can maintain your temps and then go to sleep. Definitely tap the legs to knock the ash off the briquettes to keep them burning.
Also, pork butt is very forgiving so if you do get a spike to 275-300 overnight, you are really just speeding up the cook the next morning. What temp range were you shooting for ?