* President Kiir has declined to release the detainees
* Ceasefire deal in balance over the detainees issue
* Machar worried over Uganda troops in South Sudan
By Aaron Maasho
ADDIS ABABA, Jan 12 (Reuters) - South Sudanese rebel leader Riek Machar's demand for the release of detainees remains a stumbling block to a ceasefire deal aimed at halting violence in the world's youngest state, a...