It seems that "who" is being used here as a demonstrative pronoun analogous to the German "diese"?
"Nachdem diese mit ihrem Sprecher gekommen waren, [neues Subjekt, neuer Hauptsatz]..."
I have never seen who used in this way either, but it seems it was once possible.
The same construction can be found at http://www.wishart.org/scotsworthies.html
" In his way thither, two friars came to him, requesting him to make his confession to them, which he refused, but desired them to bring Mr Winram, who had preached that day; who being come, after some discourse with Mr Wishart, he asked him if he would receive the sacrament of the Lord's Supper"
"Whereupon it was agreed to adjourn the Consideration of the present and to send for the Indians who being come and seated:
HETAQUANTEGECHTY repeating what had been proposed to them at the Conference, said:"
"her sister Johan being abroad about other business was called for who being come to her sick sister demandinge how she did she very loude and earnestly had her sister make her ready."
"They resolued to send for sir Thomas Gates, who being come, they adiured him to deale plainely with them, and to make a true relation of those things which were presently to be had, or hereafter to be hoped for in Virginia."
and finally, here in a text purporting to be Luther's:
"For the law hath his bounds unto Christ, as Paul saith afterwards: 'The end of the law is Christ' [Gal. 3:24; Rom. 10:4]; who being come, Moses ceaseth with his law, circumcision, the sacrifices, the sabbaths, yea and all the prophets. -- Martin Luther"