The 'South King', Feng Yunshan was generally considered the best of the Taiping leadership and really built up the majority of the force that rebelled in Guangxi. However, he dies following a freak cannon shot outside of Yongzhou, Guangxi in 1852, 1 year into the rebellion and 1 year before Nanjing was captured, so he might be of little use here. Also I'm not sure how friendly he would have been to the Westerners; certainly he would have been more pragmatic - a Zhou Enlai to Hong's Mao Zedong, if you will.
Hong Rengan, 'Shield King' of the Taiping kingdom and Hong Xiuquan's cousin is your best bet, arriving in Nanjing during 1859 (too late to stop the killing of East King Yang Xiuqing, or to keep the talented general Shi Dakai from leaving). Having spent the 1850s in Shanghai and Hong Kong, he knows many of the missionaries in China personally and generally is considered more pragmatic than Hong Xiuquan.
Li Xiuqing, the 'Loyal King', is also sympathetic to Westerners and led the wildly successful force that seized Suzhou and Hangzhou in 1860, though he also led the disastrous attack on Shanghai in August 1860 that resulted in Western intervention, though not because of any malice to the West on the Taiping's part. Further attacks on Shanghai and Anqing in 1862 and 1863 did not help matters.
Honestly, however, it's going to be hard for the Taiping to negotiate with the west. I think the closest analogy to the Taiping was a jihadi state - no peace until everybody in China was converted. This was not what the Westerners wanted. But if pressed, an administrative team of Feng Yunshan/Hong Rengan, backed up by the military talents of Li Xiuqing and Shi Dakai could have worked. More importantly, the Taiping must not attack Shanghai - for attacking Shanghai causes the Westerners to ban arms exports to the Taiping, fatally weakening the latter's forces against the Qing Dynasty.