When Public Forum is broken before the honor token is placed on it, does the opponent get the choice to discard the card in the provenience?

No, because the honor token is placed on the province instead of breaking the province. So all province breaking effects and requirements do not happen if Public Forum is triggered. Notice it is an interrupt and so happens before reactions to a province breaks have a chance to occur.

Why is the wording 'would be broken' used instead of 'would break'? I feel like this should be an interrupt to the 'breaking' of the province and not the status of 'being broken'. Does breaking a province occur prior to a province 'being broken'? If so, can reactions still occur to the province 'breaking'?

It looks like step 3.2.5 includes both breaking a province and a province being broken (is broken), so it's likely interchangeable.

1 hour ago, LuceLineGames said:

Why is the wording 'would be broken' used instead of 'would break'? I feel like this should be an interrupt to the 'breaking' of the province and not the status of 'being broken'. Does breaking a province occur prior to a province 'being broken'? If so, can reactions still occur to the province 'breaking'?

Active vs Passive Voice.

"I threw the ball" is the same as "The ball was thrown (by me)."

"I broke the province" is the same as "The province was broken."

Glad to see those years as an English major are really paying off.

1 hour ago, twinstarbmc said:

Active vs Passive Voice.

"I threw the ball" is the same as "The ball was thrown (by me)."

"I broke the province" is the same as "The province was broken."

Glad to see those years as an English major are really paying off.

Yep. Guess whoever wrote the card text didn't have "avoid be verbs" drilled into them hard enough.