@John - that is some excellent advice! Thank you. Sort of learning and fighting with similar issues. We also brought in a kit with an adjustable fixture and a mix of different diameter solder balls. While trying a thin metal stencil for memory chips, the heat from the gun would warp the stencil and make a mess while lifting the melting paste. From what we know so far, not every stencil is meant for direct heating. The direct heating style of stencils are more rigid under the same temp of the air gun.
Anyways the trick you noted to heat from the 'opposite' side is gold
Thanks again. Will try this next week.
PS: To the OP - the solder balls are available as usual for low costs from Asia on Aliexpress. I think we are using 0.3 or 0.35mm balls so they are not applicable to you. If you are really in a pinch, ping us and can see we can spare our 0.5mm balls or you can just replace them when you have them from your source. We are in Windsor, Ontario. Also, consider to purchase a small bga with a similar geometry to perfect this process.