wow, so a new election schedule is huge! and it seems that foster would only have a few months in office.
still, do we know if he was likely to push on civil rights, or go easy on the planters?
Why not both? Many people did not see the two as imcompatible. Horace Greeley, for a famous example, spoke of "universal amnesty and impartial suffrage", and Senator William M Stewart of Nevada introduced legislation of a similar spirit in 1866.
Regarding Foster, he seems to have been over on the moderate to conservative side of the Republican Party, so there won't be anything
really radical, but that wouldn't necessarily exclude measures like the enfranchisement of Coloured Union soldiers, or a requirement that any literacy or other tests for voters must apply to whites as well as blacks. About the same as might be expected from Lincoln had he lived.
And, who now is likely to win the emergency election?
It will certainly be a big name. In this situation, all will be looking for a figure who commands public confidence.
If Grant is still alive, then he gets it for sure. If not, then probably Sherman. I know Sherman wasn't keen on politics, but in a situation like this, he would probably feel obliged to run, if only to stop some weirdo like Butler from getting it.
Small point. Whoever it is needn't necessarily wait for the election. There is no constitutional requirement that the President of the Senate be himself a Senator, so that body could at any time choose Grant (or failing him Sherman) as President Pro-tem, and have Foster step down in his favour. In practice, they would probably wait until after the party nominations had been made. Iirc, Foster's later career was mainly as a judge, so he is probably rewarded for his co-operation by giving him the next Supreme Court vacancy.