Is it essential to glue the filter plates to the base glass?
No, the weight of the filter bed tends to seal the plates from any leakage.
Can air be used to operate the uplifts?
Yes, but powerheads perform the task much more efficiently and just as cheaply.

Can i put a layer of floss over the filter plates as gravel tidy substitute?
The floss will become clogged and impenetrable to water within a few months. Replacement means major disturbance of the whole aquarium on a regular basis.
How can the media be checked for contaminants?
A strong magnet placed inside a polythene bag can be passed over the media to remove any metal fragments.
What will happen if two different powerheads are used for the same filter bed?
The stronger pump will reduce the flow of the weaker one as it pulls against it. In extreme cases, water will be drawn down the weaker pump, rendering the whole filter bed useless.