Big-bubble soapy looking head I would generally attribute to undercarbed beer (or just barely reaching proper carb level if in a keg), but after 6 weeks in a bottle I would expect more head especially with the amount of hops you used.
The glass is a potential that's easy to remove from the equation. I actually wash my glasses either in the dishwasher or by hand with dish soap, and I don't suffer from head retention problems.
I've never used carb drops so I can't comment directly to their relation to head retention, but they are worth researching more fully to determine if they are a potential culprit.
As far as the extra 30 minutes of mashing, while there is some subtle protein degradation processes still taking place at saccrification temperatures, I would not expect 30 additional minutes to cause you this kind of trouble. Afterall, some folks will mash overnight (8+ hours), and don't experience a totally lackluster head like you've mentioned.
In regards to head retention from bottle conditioned beers, I've always found them to be superior to keg carbed beers. For whatever reason (call it "natural" processes), bottle conditioned beers usually produce wonderfully full and billowy heads - sometimes in excess - that can last decently long. If I were you, and I'm not, I would try using the standard priming sugar method of carbing your next batch (i.e. corn sugar instead of carb drops) just to see if there's much difference.
(but I DO think some folks think that way 
