p.1 #2 · Flash storage: keep capacitors charged or discharge for storage?
You don't want to leave batteries in devices in long term storage because they may leak and damage the device. Beside they will do no good if the flash is off.
Flash capacitors have a tendency to becomes "deformed" if not used for extended periods of time. Once deformed they will not accumulate a charge, so it is a good practice to take them out of storage every month or so and run them through 10 or so power cycles, then take out the batteries with a full charge left in the capacitors and store them again. While probably not an issue with speedlights for larger studio flashes it generally is better to start at lower power levels then raise them as the unit is cycled. It won't make it any less likely to fail, just minimize damage if it does,
WIth a unit that has been in storage for some time and doesn't appear to charge patience is a virtue. Some years ago I reformed some Vivitars that had been in storage for a few years and when initially trying them one appeared to be dead. But based on advice I read I just kept them under power and swapped several sets of batteries over several days finally the ready light came one and it worked normally again.
p.1 #3 · Flash storage: keep capacitors charged or discharge for storage?
The precautions Chuck stated are for longish-term storage--maybe a few months with no activity.
If you're only talking about a couple of weeks, you don't need to take any specific precautions (other than removing the batteries, which can sometimes perversely decide to leak in only a few days).
In any case, it's really much better for your flash units to make sure you exercise them at least every few months. The capacitors are semi-dry chemical devices, and like sealed lead-acid batteries, they need to be exercised occasionally to maintain effectiveness.
p.1 #4 · Flash storage: keep capacitors charged or discharge for storage?
Doesn't matter. Unless you're the Flash (the superhero, not the photo equipment), it's going to recycle most of the way before you can hit the power switch. And even if it does, Speedlights lose charge very quickly. Whether you put it away fully charged, partially charged, or fully discharged, it'll be discharged shortly.
