How does one make use of a Replacement Battalion? For example, to account for combat losses among other battalions?
So far as I can tell, a Replacement Battalion is just a really crappy unit, probably from being only barely trained.
This is an incorrect assessment of replacement battalions.
As someone mentioned upthread, replacement battalions make depleted units return to full strength more quickly. You should be able to find the thread in which Steve introduced them and discusses them, probably in mechanics and probably with a title that's talking about "new" ground combat rules.
John