Thai is available for
luatex, but the
preferred engine is the latter. With
luatex, it should work with the
default font renderer and Harfbuzz.
Rules are harcoded in XeTeX, but in LuaTeX the standard hyphenation
mechanism is used, son that you can load different rules or fine tune the
loaded ones with
\babelpatterns (or even a transform).
babel attempts to get rid of hyphen chars at the end of
lines, but due to the limitations of this engine it sometimes fails.
With LuaTeX there shouldn’t be any problems.
Justification in both engines is controlled by a couple of options in
intraspacesets the inner spaces in em units (so,
0 .1 0is 0em plus .1em). Like
\spaceskip, the em unit applied is that of the current text (more precisely, the previous glyph).
intrapenaltysets the inner penalty of the breaking opportunities.