Here we go again. Let's see if someone wins this time around.
Shorter lead to throat—maybe. Just don't shoot the heavy/longer rounds. 55gr fmj/bt/ will be fine.
It turns out that NATO 5.56 is more pricey now anyways, so why shoot it in .223 when there are more accurate, cheaper .223 offerings.
.308/7.62—This is different and significant I believe (of course I might be wrong). It depends on origins of the barrel. Some European 7.62 projectiles are "correctly" dimensioned at .312 OD—correct because the rifle grooves are actually 7.92—and may hurt a .30 rifle made for .308 projectiles.
If you're just using the brass this is a moot point... because you have worked up your loads accordingly. Full-Length/Small-Base/Neck sizing... it doesn't matter, it's up to you and what works well in a given rifle. Do it right per the books and watch for pressure signs.