If the priest is treated by the Church as guilty until proven innocent, they do indeed face official (within the church) recriminations while the incident is being investigated.
As many of these 'incidents' are only coming to light years, sometimes decades, after they allegedly happened, some of this reeks of lawsuit lotto.
I'm not saying that no incidents ever occurred, but consider that the taint of accusation, if unwarranted, not only removes a priest from his current contact with the public, but even if proven false, will follow that priest for the rest of his life. Needless to say, that would be a career wrecker.
Before one goes calling for heads on pikes, consider that this would be an excellent way for someone bent on damaging the Church to proceed, through baseless accusations, to cripple the clergy.
There are those who just might have such motivation, because the Catholic Church will not accept nor perform homosexual marriages, nor will it sanction abortion. When one considers the people who believe their ends justify any means, the abortionists, homosexuals, and others just might employ such tactics.
With the willingness of the media to belabor any accusation, justified or otherwise, it is only prudent that the Church isolate the accused (to prevent any further incidents, if indeed wrongdoing has occurred) and perform its own investigation before calling in authorities, otherwise, the tactic would be entirely effective.
Of course, not every allegation is true, but the natural defense mode of the church is dismiss allegations as "lawsuit lotto" instead of appearing to be serious. You say the church should "isolate the accused (to prevent any further incidents, if indeed wrongdoing has occurred) and perform its own investigation before calling in authorities..." but that is exactly the problem. In many cases, the accused priest wasn't removed from ministry and still had plenty of access to new victims. This case in Philadelphia involves Monsignor Lynn who supervised 2 of the abusive priests, but failed to adequately investigate and kept those priests active.
You fail to understand that church leaders, such as Lynn are part of the problem and can't be trusted to perform legitimate investigations. They have a conflict of interest to protect the reputation of the church and not take allegations seriously. The church would have more credability if they immediately involve a third party, such as the police (a novel concept), to perform an adequate investigation.
To some extent, someone accused of some crime IS treated as potentially guilty until a trial. One is presumed innocent until proved guilty. They cannot be jailed without a trial establishing guilt.
However, for an investigation to proceed, there is going to have to be some kind of presumption of guilt and inconvenience to the person, innocent or not. There is simply no way to investigate a case and look for evidence without some kind of presumption of guilt. What the law does is protect the accused in the meantime and prevents the accuser from acting on only a presumption.
When these incidents came to light, the priests should have been immediately removed from their position while a complete and thorough and RAPID investigation is made. People in other situations where they are being investigated for some kind of crime are sometimes put on administrative leave while the investigation is ongoing. Sometimes life isn't fair.
If his innocence is established, restore him to office. If not, throw his sorry butt in jail.
But to shift those men accused of impropriety around to cover for them and protect them while they continued to do it and not investigate it further, is unconscionable.
Sadly, the Roman Catholic church has a LONG and sordid history of sexual misconduct, easily going back a thousand years. That does cut into their credibility more than a little.
If they don't like the bad publicity, they need to remember that they brought it on themselves with the way this stuff was handled. If they don't want the criticism, man up and do the right thing.
Nobody will criticize them for acting appropriately when the situation comes up, as it is bound to now and then.