Here's my idea:

Decline a single event in a recurrent (weekly, monthly, etc) event +
textbox for a reason

If I have a weekly event on egg. Monday 10:00 and I want to only decline
the event for next Monday, Business Calendar always declines all events
if I check the "no" radio button on "Are you attending?"

Could you please check this and fix it if it is broken?

Maybe there should be two options:

"Are you attending this event?" (YES, MAYBE, NO)
and
"Are you attending all recurrent events?" (YES, MAYBE, NO)

If you choose not to participate on an envent on the google web calender,
it is asking you if you want to stop participating in this or in all future
recurrent events. Also it lets you add a reason why you do, don't or maybe
attend.

Thanks for considering my request