I answered these questions in the formal support ticket that you have opened. Again posting the answer here for others to see, but if you have any further concerns, let us continue posting in the formal support thread, so that we avoid duplicate posts. Thank you in advance for your understanding.
You can fire any grid command in code-behind by using the FireCommandEvent() method. In the specific case with filtering, you would need to get hold of the GridFilteringItem and fire the command upon it. More information is available at that address
Or if you want to capture the Filter command on the client to cancel it on some condition (like checking the command argument) you can wire the client OnCommand
event which is fired for each command before it gets executed (in this case through a postback).
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