Simple question:

May i choose to kneel my charachters to attack when To the spears is revealed or may i choose to kneel The Red Viper (pots) to attack

when my opponent controls more charachters.

The question is obviously about the theme "is Valar Dohaeris a silver bullet for those Red Viper and toys Martell decks" ?

You cannot choose to kneel characters who do not kneel to attack just because you want them knelt. Like any passive or constant effect, they are always there and you must always deal with them. You cannot ignore them just because it would be convenient or beneficial to do so.

So yes, Valar Doheris does mess with the "does not kneel to attack" mechanic quite a bit.

Thank you for your answer Ktom.

I guess Valar Dohaeris will find it´s way at least in my Baratheon deck now - it works nice together with Black Cells.