> Taldi Leifur nú mjög á hendur förunautum sínum
> Leif greatly reproached the hands (actions ?) of his followers

That's the idea: he blamed them (literally, their hands).

> sinn dag hvort

He's suggesting that they alternate the work, picking grapes on one
day, then chopping vines and timber the next, then grapes the day
after that, and so on. See the comments I've just posted on Grace's