From what you are describing it does seem most likely that the battery has carked it but is holding enough surface charge to work OK if not loaded heavily. The extra 15+ amps being puled by the lights is knocking it down to the point it doesn't want to crank properly. Which means you are on borrowed time - carry jumper leads and/or a booster.
A mate of mine put me on to this gadget recently -
12V Car Battery Monitor via bluetooth 4.0 Voltage Meter Tester w/ auto Alarm | eBay Being the boffin I am I just had to get it...
Amongst other party tricks it shows you the battery voltage across the terminals in real time, wirelessly to your phone. And can do a charging test at the tap of a button. During cranking it measures and records the voltage every 10ms (a hundred times a second). Refer attached pic for a result I got a few days ago on the very tired battery in my well-worn V70 showing that the battery is pulling down to much less than it should during a cranking event. This was after standing for only three days and being fully charged with an AC-DC smart charger at that time. It's also showing that my alternator is barely pushing 13.8V into the battery and that degrades to 13.5V after an hour's running as the alternator gets hot and loses performance. That is not enough to properly charge a battery, part of the reason it's dying... The car has done nearly 400k kms so it's allowed to have a couple of minor issues. All I've done is replace the brushes, and that improved things, but not enough.
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Best of luck. Hopefully they'll stump up a new battery to replace the current one under warranty.
P.S. How are you finding the Glohh conversion? Latest GL3 (dynamic) type? How good are the reverse lights and how visible are the lights generally compared with standard? Money well-spent, or bit of a luxury easily avoided? :P
