Tefilin - first parsha only

On Friday morning I started writing parshiot for tefillin shel rosh. I didn't anticipate having any problem completing at least the first two, but it both took a lot longer than I expected and I had to stop twice to wait for letters to dry so that I could correct them. I ran out of time and was only able to finish the first parsha.
All the sources I've seen say that you need to write at least the first two in one sitting. What is the status of the parsha that I wrote? Is it kosher but not lechatchila? Should I start from the beginning but save it in case I ever need a first parsha written at an earlier date? I saw the Lishkat Sofer brings down an opinion that if there is a sofer who is not robust and will likely make mistakes if he writes for long periods, that it is better to take breaks. But I could not be certain if he was talking about breaks after finishing the second parsha (as opposed to writing all four at once), or breaks in general, and whether he is just talking about brief pauses or more than that.

And while on the topic, what are you supposed to do if you're writing tefillin and you make a mistake? Is the correct thing to sit by yourself and wait for it to dry without talking? Start another set of tefillin so that you stay in the same mindset? Or is it acceptable to take a break and come back to it?

Thanks.

Josh Skarf
Jerusalem