Just an idea, I have never done this, but - you may want to check out "chip quik" - Using chip quik you likely could prevent the components on the back from coming off by lowering the temperature of the top components quickly using hot air, once it mixes in then the difference (in melting temperatures, if you're careful) will be large.
GET OTHER PEOPLES OPINION ON THIS!
Another idea, blow a cooling fan across the bottom while heating the top with hot air. But that could also cause problems, with mechanical stresses.
I often replace USB sockets and other components in mobile phones, and have used low melt solder with great success. There are some issues to be aware of:
Most low melt solder does not contain flux, so this must be added separately.
The low melt solder must be mixed with the regular solder, which means: Solder ALL the joints with a normal solder iron, and make sure it reflows.(This ensures that
if the connector has a ground pad underneath it, you do not lift (hopefully) any pads when removing the defective plug/component from the board, when doing the final removal procedure)
When successfully reflowed, the solder is workable for a much longer time, at normal solder iron temps - and some times hot air is not needed to remove the item. It depends on the ground plane of the PCB.
Hot air is great, as it heats the PCB in general, including the ground plane - and reflowing with low melt can enable you to use lower temps, thereby not melting the solder of nearby components.
- making removal quite easy.
Patience and experience is key, so do some practicing on scrap boards to get to know the mechanics of soldering, the reflow temp of solder, the time to heat up ground planes, the force to apply, etc.
Remember to wear googles/eye protection, if the component sticks - and you suddenly jerk it off by mistake from applying too much upwards force and not waiting too long, as molten solder is HOT - just a kindly reminder
Low melt solder must be removed after use, as the alloy is not as strong as regular solder!Cleaning with iso-alcohol, and applying a line of solder paste (experience here is great, a thin line and a dab in the holes - and hot air reflows the components perfectly, and it looks as new)
Using a microscope can be very useful for tight pitched components, to ensure the right amount of paste is applied. I use tweezers to move the balls around, and to ensure the flux is wetting all pads.
Btw - most low melt solders differ, and some contain hazardous metals. I recommend getting solder where you know the 'secret sauce' like
http://www.ebay.com/itm/Solder-wire-low-melting-point-124C-1m-1mm-Bi55-5-Pb44-5-alloy-good-desoldering-/171173385076Capton tape is your friend, and an ir-preheater can also be great on multilayer boards.