Here's the scenario:

I attack with the no-kneeling Knight of Flowers with a Hunting Spear (reduces non-weaponed characters -2).

A few questions:

Can I also declare 2 STR characters, even if they will have no strength due to the HS? ( for their renown or character ability, etc)

Also, if a character's STR is reduced to zero during the challenge, they are still participating, and could still claim renown as long as there were other participating characters that would win the challenge. Right?

Short version: can I declare a 0 STR character in a challenge?

There's nothing preventing a STR 0 character from participating. Incases like Hunting Spear or Intimidate, you may often want to have additional characters participating to trigger their abilities.

To win (or to oppose a challenge) you have to have at least 1 total STR, but that doesn't even have to be from a character (Eg a STR 0 character with +3 to your side from a melee title is sufficient).

Thanks for the response.