To this we may add that as the CSA coalesced, the Southern idea of re-entering the Union on... new terms... faded out rather quickly.
However, ISTM that the victory at First Manassas truly confirmed that coalescence. It appeared to validate all the gasconading of the Fire-Eaters, and provided a big (IMO critical) psychological impetus.
If the battle had instead been a decisive Union victory... A lot of voices would be heard saying the entire proposition was reckless folly. I'll agree that Lincoln would under no circumstances negotiate the
de jure "return to the Union" of any state. But he could be flexible, as when he offered to trade "a fort for a atate". He would not insist on explicit formal admissions that secession was invalid.
Suppose Tennessee, for instance, passed a resolution declaring that secession was rescinded, recalled Tennessee troops in CSA service to be disbanded, and sent Representatives and Senators to Washington (including Andrew Johnson). Would Lincoln object to their being seated? The Tennesseeans also ask Lincoln to appoint US marshals and postmasters for Tennessee, providing him with a list of preferred nominees, i.e. ex-Whigs and Unionists. Would Lincoln reject this tacit acceptance of Federal authority and his election?
There could be other conditions, agreed to in
private discussions: amnesty for all those involved, for instance - or nearly all. IOW, negotiations, but nothing official.