In answer to your original question, no I don’t think complete pan-Protestant unity is possible, thoug some stabs have been made at it in the Ecmenical Movement,things like the Methodist church merging with the United Brethren in the United Methodist church, the recognising of Orders and Sacraments between the Episcopal and ELCA Lutheran churches, and best of all the combing to two differing theologies Lutheran and Cavinist-Reformed in hte United Church of Christ.
But those uniting churches are Main-Line relatively “liberal” churches.
The story with the evangelical/fundamental churches will be very different. Before I became Catholic I was in the “church of Christ” and they will not bend at all, even though ironically they say they were formed to bring unity to all Christians their path to unity is for everyone to abandon their original churches of birth and just join them, that way everyone will be united. And the proselytizing is bad too (which is very different from Evangelising).
The church of Christers and Baptists are still fighting like mad, stealing each other’s sheep.
The more conservative churches seem to be stuck and only interested in uniting on their own terms.
For example the hyper-conservative Lutheran Church Missouri Synod will not even give other Lutherans Holy Communion, not to mention anyone else.
I think the differences between just Protestants are insurmountable. I mean their are major differences. Holy Communion weekly against totally symbolic “Lord’s Supper” four times a year, Calvinism as opposed to Armianism, Baptism of small chidren VS beleivers baptism and only by submersion.
I think the differences are too vast to expect even Protestant unity.