This keeps cropping up from time to time and I would like to know for certain.

Do I need to stand The Knight to trigger the discard? My understanding is no, as these are two

separate conditions and standing doesn't seem to be the cost for triggering discard.

That also means that when someone blocks his response he stays knelt, unlike blocking To Be A Kraken.

Is this right or do I really need him to stand before triggering the second part of the response?

Threat From The East seems to be suggesting that he needs to stand for the second part to kick in.

The key here is the word "Then". It has a special meaning in the formal language that is used for AGoT card effects. It means that all effects before "Then" need to resolve successfully and completely for the part after "Then" to happen at all. If The Knight doesn't go from a kneeling to a standing position, no discard will happen. If The Knight's Response is canceled, the whole effect needs is canceled. The standing doesn't happen, ergo the discard doesn't happen either.