if you are in byos your heals are checked in your spell bar top to bottom, so if you have your CH earlier in your bar than your light line, its going to cast the CH first.
loaded up, byos'd - have my healshealsnotma set appropriately, casts the light line as expected.
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I would suggest you double check your healing thresholds
to answer your question directly "is there a setting for mq2cleric to use the light spell line?"
no. it already does use it if you have it memorized.
Non-byos it checks it before CH - because we're checking those spells directly.
in BYOS it checks it based on your spell order in your spellbar, this allows you to use any healing spells in your byos.