Add (to your ptxprint-mods.tex) the following line:
\interlinepenalty 10000
Which should (I tested it) disable all paragraph breaking, including where you want it because the page length will overflow otherwise. You could thus do something like this instead:
\def\NoSplitParagraphs{\interlinepenalty 10000}
\def\AllowSplitParagraphs{\interlinepenalty 9999}
\NoSplitParagraphs
and then add some changes.txt rules that do things like:
at ROM 94:12 "\\p" > "\\p\\AllowSplitParagraphs"
at ROM 96:1 "\\p" > "\\p\\NoSplitParagraphs"
(You cannot put \interlinepenalty 1000 directly into the USFM, because TeX is told to treat numbers as letters in USFM, in order to allow \s1 etc)