Can we meditate on the Guru while chanting God's name?
Yes, you can. God-realisation is possible through your exclusive devotion to Guru and by revering God and Guru equally in your devotional practice. Devotion to God alone is not recommended. Thus, while meditating on Guru, keep the form of God in your heart and mind as well and practice meditating on Him. While chanting the glories of Shri Krishna's names and His various divine pastimes, including those of His childhood, you must incorporate God's visualised form in them.
For meditating on Shri Radha Krishna, you may take the aid of a picture depicting Them and then close your eyes and recreate that image in your mind. Then again open your eyes, look at the picture once more and then close your eyes to recreate the image in your mind. Keep practicing this regularly for a month or so and then you will be able to do rupadhyana. Therefore, there is no need to worry.
Even if you always just keep remembering Guru exclusively while practicing rupadhyana, God will be all the more happy with you.
Doing rupadhyana of Shri Radha Krishna during sankirtan or replacing Their faces with the Guru's or doing rupadhyana purely of the Guru, which is best?
Whether you meditate upon the form of your Guru or Shri Radha Krishna the result will be the same. It is not that meditating upon your Guru will grant you some greater spiritual benefit. However, it is far easier to meditate upon the Guru because you have seen him on so many occasions. In comparison, you have to employ all your faculties and work hard to imagine what Shri Krishna looks like. A mere glimpse of your Guru or hearing a few simple, yet profound words from him like, "You are mine," or receiving a small gift from him are all enough to trigger the flow of divine feelings in your heart.
Original answers given in Hindi by Jagadguru Shri Kripalu Ji Maharaj