I started it in the Day and Hour of Venus, but the final affirmation was in the Hour of Mercury.Did this mess it up or weaken the Square significantly?
I suggest you start on the day and at the time of the square, it's fine to do it at different times on other days if it's not possible to do it at the exact time.
It's fine. What matters is when you start doing the spell rather than when you finish it. Let's say you begin to do it 5 minutes before the Venus hour ends and you finish doing your affirmations in a different planet hour, it does not matter because you started it in the Venus hour.
I suggest you start on the day and at the time of the square, it's fine to do it at different times on other days if it's not possible to do it at the exact time.
It's fine. What matters is when you start doing the spell rather than when you finish it. Let's say you begin to do it 5 minutes before the Venus hour ends and you finish doing your affirmations in a different planet hour, it does not matter because you started it in the Venus hour.