You raise some very good points, some of which I really should have thought of myself, given that in the Stephanie Crowe case that I mentioned, LE thought they had a mountain of evidence to use when they made the arrests, including hair in the poor girl's hands, but when the tests all came back, none of it was usable against their suspects (because they didn't do it, it turns out)...which again makes me wonder why they bothered arresting him on the murder charges before any of the final results of testing would have been back (I'm sure much of that isn't in even now, these things take time), given that he really wasn't going to be out of custody for a good long time, between house arrest and then the charges that were already against him...not too surprised though, this gets people off of the chief's back...
I'm still not sure on the multi-perp thing, but I do hope that LE really does have a good reason to believe it was a solo job - otherwise they could be really screwing up by not continuing to search for additional suspects (just like in the Crowe case)