If you have an assault marine in melee with a horde and a devastator marine attacking the horde at range with a heavy bolter or grenades, is there any chance the ranged attacks could hit the assault marine? Not hard to imagine this wouldn't be an issue with a magnitude 50 horde, but what about when the horde magnitude dwindles down to a small number like 10?

I think it's fair to impose the shooting into melee rule regardless.

That's an excellent way to deal with this situation, thanks!

The book suggests the possibility of any misses possibly transfering over to the player in hand to hand.

When my group used it we usually had about 2-3 Degree of Failures worth of shots hit the ally. Meaning a straight miss with under 3 DoF hits the ally but more than 3 DoF misses everyone. Semi and Full auto rules apply to the DoF.

That's a good process too, thanks!