This is pure hand waiving. Can you imagine the shit storm that would ensue should a former US President be arrested overseas? US Presidents are provided Secret Sevice detail for the rest of the lives. Bush cancelled his trip because he doesn't want to make diplomatic trouble for the US, not because he was afraid of being arrested. There is not a single government in Europe that would dream of detaining a US President, past or present.
IIRC, Clinton is the last US president that will receive lifetime Secret Service detail. Bush Jr. is limited to protection for a span of 10 years after his service.
Wasn't aware of that, but I don't think it changes anything. I don't want my view to come off US-centric either. No one is going to detain/imprison Tony Blair either. Nor Angela Merkel. Nor Jacques Chirac. Nor Gerhard Schroder. I would posit that the indictment of any head of state -- present or former -- belonging to a superpower would involved the explicit action of said nation, or at least the full cooperation of many other nations bringing extreme diplomatic pressure on the government. You don't just snag them while they're on a trip to your country.