The battery will power all the lights except the headlight when the bike isnt running. Yes, they will seem a touch dimmer if your battery isn't holding a good charge. I run an 1157. Yes its 12v but it helps soak up the extra juice if you have a weak battery rather than blowing your taillight bulb. It doesn't affect the headlight.
I have extracted the stock qt50 sealed beam bulb and replaced it with an auto headlight bulb single element to get by before. Normally I just repair the sealed beam headlights with a new 25/25w 6v bulb. So you have a factory headlight. Not something that fits janky and rattles.
Rear blinkers, take off the lens and there will be a 14mm nut in there. Loosen that and re-tighten it. Normally on a qt50 that's been sitting, that's where you will lose your ground.