No Hoppy,
Fuel is Jettisoned. Thats why Boeing have the "Fuel Jettison" QRH check lists.
The only dumping that takes place is when the skipper takes a physiological break.
Boeing recommend not to Jettison in the hold and caution descending at the same time.
Myth 2 seems sensible but then you shoudnt be in a thunderstorm at any time.
p.s 4000ft AGL limit from the manufacturer. We use 5000ft min