I appreciate brake safety but that's a little melodramatic. With this kind if failure it's not going to show itself catastrophically like that, it's going to progressively lock the brakes at which point (I'll know in a month or so) that it needs to be redone and I have total confidence they will honor it. I wish they would have done it all at once too.
No, it won't progressively lock the brakes. As the seals swell (they will become almost 3X their original size) they push out of their grooves, become VERY soft, and LEAK.
After this is all over, take a left-over used seal from this or any other car/truck and drop it into a pan of ATF. Measure it and take a picture first. Then wait 3-4 days and do it again.
If it were me, I would have made them start from the beginning and replace ALL of it. They could flush the hard lines as described, but the risk is too much.
Have them sign a waiver that if your brakes EVER fail, they are responsible and will cover all costs and liability, and will replace all parts damaged and required. I just hope it doesn't include people.
It may sound over-dramatic, but it is VERY dramatic, when you think all is well, but you have that little shred of doubt always on the back of your mind, about that time that you are rolling up on a school bus full of kindergartners and Nuns.
Good Luck! I feel you'll need it on this one.