Regarding your point #2...people bring this up a lot and it always sounds like a great theory that explains everything until you remember that Indiana has a legal mechanism called accomplice liability. This is different than "aiding and abetting," or assisting a criminal (harboring, helping, or giving a false alibi).
Accomplice liability means that individuals can be held fully accountable for a murder if they were an active participant in any part of it, regardless of whether they were actually the one who ended the victim's life.
In Indiana a person is responsible for the actions of another person when, before OR during the commission of a crime, he or she knowingly helps another person commit it. See Indiana Code 35-41-2-4 "One who...aids, induces, or causes another person to commit an offense commits that offense....an accomplice can be held criminally liable for everything which was a probable and natural consequence of their common plan."
All of this is to say, if LE in the Delphi case knew that two individuals were involved but didn't know which one actually ended the girls' lives, nothing would hold them back from arresting and charging both individuals equally.
There are several Indiana murder cases that show accomplice liability applied but here's one if you'd like to explore this:
Hauk v. State
To your point about circumventing a solid alibi. People act like an alibi given by a second individual creates a miraculous barrier that prevents LE from charging someone and trying them. IMO nothing could be further from the truth. Cases go to trial with alibi witnesses called all the time and it's up to the prosecutor to show that the witnesses are not believable and why.