Confederate officers like Patrick Cleburne relatively early on seriously proposed emancipation to raise the manpower the Confederacy needed to secure its independence. Cleburne's proposal met with very strong resistance in Richmond, it rasied some real heat. cleburne and other oficers like him were more or less ordered to remain silent on the issue going forward, although Davis and Lee and others eventually came around to the notion far too late.
Cleburne also had been a British citizen and soldier, he might have made a very strong diplomat had his idea of emanicipation been accepted. To the British citizenry and diplomats and policymakers, slavery was a major issue which strongly helped prevent Great Britian from openly recognizing the CSA.
Under the circumstances, a policy choice was clearly made early on not to even consider exchanging emancipation for independence, so the phrase "War for Southern Independence" loses any real meaning from that point on, it clearly became "The Slaveholder's War".
When it could have changed everything, the leadership of the CSA faced a clear choice, and that policy choice was made -- to the leadership, maintaining slavery was clearly more important than securing independence.