You either have proof or you don't. In Trudeau's case he did not have proof, he only had an allegation and an ongoing investigation.
So he should have persisted through diplomatic channels than pursuing this through the court of public opinion.
India has zero obligations in this case to cooperate with the Canadian government who made an accusation against India, expelled a diplomat etc. That is not asking politely.
If Canada wants they can share specific information with India which India can react to. Otherwise they can work on it on their own and present evidence for their claims. Failing which, they would have failed.
No, it's an investigation. If the investigation leads you in a direction you follow it. The accusation was made by the US. Canada is just following the lead and didn't accuse anyone, only asked for assistance.
It's up to the Indian government to co-operate and assist in the investigation. They are not obligated, but that's a pretty crooked and questionable move to not help investigate a murder.
Why would they release key information on an unsolved case, especially to a government that openly won't co-operate?
And honestly, if Canada asks India about any involvement in an assassination and India's response is "fuck him, he's a terrorist", That basically saying yes we did it.