Srila Prabhupada says in the book, Teachings of Lord Chaitanya, that when we chant Hare Krishna, Hare Rama not only do we invoke Krishna and Balarama but also Lord Chaitanya and Nityananda. You said that when we chant japa, Krishna is present in His name and so we are in the presence of Krishna, speaking directly to Krishna. Can I say, therefore, that Lord Chaitanya and Nityananda are also present in front of me when I chant japa or kirtan, etc and, thus, can I pray to Lord Chaitanya and Nityananda (at the time of japa) to mercifully help me chant japa more carefully and with more concentration?

Undoubtedly, Gaura-Nitai are present when you sing the Mahamantra. They are very merciful and only by His mercy, manifested in this world by Srila Prabhupada, we can overcome the ocean of Kali.