How else was emancipation to have any practical meaning?
I'm not absolutely sure what that means.
As of 1860, there were eighteen states which had either abolished slavery or never had it. Of these, only six (five New England states plus NY) allowed any Blacks to vote [1]. Does this mean that the abolition/exclusion of slavery from the other twelve was somehow "without practical meaning"? Very few people would have seen it that way at the time.
We saw OTL what happened when the reformers both north and south threw up their hands;
Isn't that putting it the wrong way round? Surely it was more a case of the reformers throwing up their hands because of what had happened, rather than vice versa. Like the British after Yorktown, they "threw up their hands" because they had lost, and had no realistic hope of reversing the defeat.
why do you think, once Lincoln realized the need for imminent emancipation, he so quickly came around to supporting expanding the franchise?
For much the same sort of reason that the Framers of the Constitution sedulously avoided using the word "slave", always going for "other persons" or the like. The institution existed, but in writing their document many of them tried to look forward to a day when it wouldn't.
In the same way, Lincoln knew that there was no near term prospect of Blacks being accepted as the political equals of whites, but he wanted to leave the door "open a crack", so that if/when public opinion had moved far enough, there would be a precedent to invoke.
[1] Seven if we count Ohio, whose definition of "white" as less than 50% black let in some mulattoes who would have counted as "negroes" in most states. NY allowed Blacks to vote if they had $250 worth of property, a condition not required of whites.