Your question, "if in the Vaikunthas ecstatic love between Lord Narayana and His devotees and Lord Krishna Consciousness and His devotees on Krishna Loka is qualitatively the same, then what is the advantage of following the path of Krishna Consciousness, whose philosophy austerity is difficult to understand?" Krishna Consciousness philosophy includes the whole Vaisnava philosophy. When we speak Krishna, the very Word means, Rama, Nrsimha, Varaha, and all other similar expansions and incarnations. So Krishna Consciousness includes Vaikuntha consciousness, but Vaikuntha consciousness does not include Krishna Consciousness. Those who are strictly following the rules and regulations of Vaisnava activities, they are promoted to Vaikuntha lokas. But those who have developed a spontaneous love for Krishna, they are promoted to the Krishna Loka. Therefore, although Krishna Consciousness includes Vaikuntha consciousness, pure Krishna Consciousness is spontaneous love for Krishna without any regulative principles. The idea can be explained that a young boy or a young girl are spontaneously attracted, without following any regulative principles previously. When one develops such spontaneous love for God, that is called pure Krishna Consciousness. In our conditioned life, we have forgotten our relationship with God, but by regulative principles, we can make alert the inert activities. Just like a rheumatic crippled man is gradually elevated by some bodily exercises. Similarly, the regulative principles are to make us habituated to our dormant service attitude, but when that is mature, it becomes spontaneous, and that is pure Krishna Consciousness. And such highly elevated Consciousness makes one eligible to enter into the Krishna Loka. The living example for much spontaneous love are exhibited by the Gopis and inhabitants of Vrindaban. We can learn this from Srimad-Bhagavatam how much they love Krishna spontaneously.
Your question, "what is the relationship of sweethearts in Vaikuntha to each other and to Lord Narayana? Is conjugal love, parental affection, and unalloyed friendship there in the Vaikunthas?" No. In Vaikuntha there are two kinds of transcendental mellows, dasya and the lower half of sakya rasa. The lower half of sakhya means friendship in adoration. And the higher half of sakhya rasa is friendship on equal level. So in Vrindaban the reciprocation of transcendental humor is higher than in Vaikuntha. It is there more free and spontaneous without any restriction arranged by yogamaya principle.