Hi
For question 2 : you need to make sure the factory applies the Customer Process Guidelines (available on the Sierra Dev ZOne) word by word : reflow profile, stencil specs. When Sierra releases an LGA module, a second reflow test (equivalent to the assembly at your EMS) is done on several thousands of units.
Besides, all modules are tested on production line before shipment, and they all should look the same. If not, then it is most likely due to the second reflow (as anticipated above)
If after checking that your EMS did strictlly apply the process guidelines, you can send the X-Ray picture to your Sierra Wireless distributor for analysis by them or direclty by Sierra.
Question 3 : Sierra distributors usually hold a stock of modules for fast delivery. Check list of distributors :http://www.sierrawireless.com/wheretobuy/distributorsandresellers/distributorlocator.aspx?pc=WS+Series
Question 1 : have your EMS x-rayed your board ? 3.7V on UART looks like more an internal short, but I can’t be 100%, assertive. Anyway, from my point of view, you should make sure that you have 100% reliable assembly processe before investigating this issue as it may be linked.
dpm