If you use a temperature hot enough for reflow (200C or more, according to andyjohnson0), is there any risk of heavy components on the bottom side falling off?
Except for extreme cases, no. The cohesive force of molten solder is more than enough to hold your components in place. Also interestingly enough, this is also what causes surface mount components to automagically "settle" over their pads when you have the proper amount of solder/flux.