I'm wondering about (what may be) a fine point of FO semantics.

Is there something about fo:inline that prevents them from breaking across lines?

I'm prepared to use fo:wrapper to instigate momentary changes, say, into an italic face -- but had supposed fo:inline was the right way.

The difference between fo:inline and fo:wrapper is that the
wrapper only holds inheritable properties for the areas
within, while the fo:inline generates areas. This means a
fo:inline can have a border, background, space-start and
space-end and padding and a few other bits, while a
fo:wrapper cannot. If you just want to change the font-style
to italic, it doesn't matter whether you use fo:wrapper or
fo:inline. The latter is more commonly used though.