--The truth, in reality, is more boring--
That's the way it usually is, anyway.
The picture that seems to fit the facts most closely is quite pedestrian.
Marcinkus was not a particularly adept financier and the odd financial conditions that prevailed in the 1970s (remember stagflation?) stumped him. He mismanaged the Vatican's finances into the red.
As is usual in such cases, he was approached by bankers who knew that he was having difficulty and who offered him several seemingly attractive ways to invest the money for above-average returns in order to make up the shortfall.
At this time in Italy the Mafia was more influential in finance than it is today and it's obvious that Signore Calvi was a banker whom the mob had something on.
He worked as a front man for the Mafia trying to find willing suckers to invest in fake businesses that were really Mafia scams.
The elderly churchman who was in dire straits and who was extremely eager to improve his portfolio bought Calvi's salesmanship and invested aggressively with him and others.
Then the losses started coming in and the Mafia found out that Calvi was skimming the Mafia's money off the top.
Calvi was killed, as were others.
The Vatican realized that it had been taken and worse, the prestige of the Church had been used by Calvi to market the scams to other pigeons, i.e. "Hey, the Vatican has invested heavily in this you know - I could see if they would be willing to let you in."
Realizing they had been taken for a ride and that Marcinkus had been duped, the Church tried to make up for the losses of others who had been scammed by Calvi by making a capital infusion so other investors could be made whole, while the Church took the full hit.
The Church felt that the anticlerical, Socialist government of Italy had put a target on Marcinkus' back, which was convenient for them, since they knew that if they convicted Marcinkus they would be praised by the secular press and be safe, while if they aggressively went after the Mafia, they might get assassinated like other Italian prosecutors at that time.
The Church refused to sacrifice Marcinkus to please the newspapers.
And for the record, the Vatican never claimed that Pope John Paul the first died from "overmedication". He had a heart attack and medicine had nothing to do with it.